Here's the article: https://inveritateblog.com/2020/03/07/the-una-cuм-mass/

It's pretty much a summary of that same set of arguments Bp. Sanborn makes against going to una cuм masses. In my view, this is Sanborn's biggest fault. I don't understand why he takes such a strict stance on it. I have talked to a priest from MHTS and emailed Fr. Cekada, both of whom are anti-una cuм, about going to a sedevacantist priest who doesn't say the Mass una cuм, but agrees that it's okay to do so. Fr. Cekada says that it's fine to receive any sacrament from this priest, whereas MHTS states that you not only shouldn't go to their mass, but you can't even get confession from them.

That is a completely ridiculous stance, especially because there is no actual precedent for rejecting a valid una cuм-equivalent Mass in the history of the Church pre-Vatican II. Rejecting una cuм so harshly only seems to divide sedevacantists unnecessarily. As Ladislaus showed me, even the Dimonds have no problem with going to an una cuм Mass. Some say they no longer do, but they haven't removed their articles defending it and have stated that they believe they received genuine spiritual graces from doing so.

This isn't to defend the Dimonds, who I don't fully agree with, but to highlight Sanborn's ridiculously hot take on an issue that shouldn't be much of an issue at all. It comes off as very intellectually dishonest to me. If his perspective was the genuine consensus of those who are against una cuм for following their conscience, I would be fine with that. But Sanborn brings it to such extremes that it comes off as power-grabbing so that more people will go to your seminary. This, especially when Sanborn is very liberal on issues like EENS... 

I have talked to a priest from MHTS and emailed Fr. Cekada, both of whom are anti-una cuм, about going to a sedevacantist priest who doesn't say the Mass una cuм, but agrees that it's okay to do so. Fr. Cekada says that it's fine to receive any sacrament from this priest, whereas MHTS states that you not only shouldn't go to their mass, but you can't even get confession from them.

Huh?  Can you re-read what you wrote here?  Something doesn't add up for me, and I'm wondering whether something was written incorrectly.

Here's the article: https://inveritateblog.com/2020/03/07/the-una-cuм-mass/

The article, sad to say, is fit for the sewer. Always avoid reading things like this so as to avoid being familiarized with this thinking yourself. I suffered through the whole article and it is a fine example of a twisted mess of confused thinking. It is absolutely incredible to me to see what has become of this person whom I knew when he was a courageous young Catholic priest. 

I think what he’s saying is that the Sanborn group is now saying that you can’t even go to a sedevacantist priest who thinks that a sedevacantist can go to an una cuм mass, if they so choose (i.e. CMRI ?). I’m not sure if this is true, so I would like to have evidence for this change in position.

I had a conversation on phone with Fr. Despòsito, one of the more prolific MHTS priests. I am certain- even having asked for clarification- that he told me that I should not go to the Mass of a local sedevacantist priest who works with an SSPX resistance priest in the same chapel (Fr. Ringrose of St. Athanasius, for the record), and I'm pretty sure he said not to confess to this priest as well. When I asked him about CMRI, he said one could go "at their own risk."
I don't know if this is just his position, but I believe he was speaking for the seminary. If anyone else wants to ask him, I know he's pretty active on social media, and I have his phone number, although for obvious reasons I will not be posting it publicly.

I had a conversation on phone with Fr. Despòsito, one of the more prolific MHTS priests. I am certain- even having asked for clarification- that he told me that I should not go to the Mass of a local sedevacantist priest who works with an SSPX resistance priest in the same chapel (Fr. Ringrose of St. Athanasius, for the record), and I'm pretty sure he said not to confess to this priest as well. When I asked him about CMRI, he said one could go "at their own risk."
I don't know if this is just his position, but I believe he was speaking for the seminary. If anyone else wants to ask him, I know he's pretty active on social media, and I have his phone number, although for obvious reasons I will not be posting it publicly.

Yes, they do have this notion that heresy and schism are contagious, that someone who is in communion with someone who is in communion with someone who is in communion with a schismatic is in fact a schismatic.  This leads to a bizarre view of the Church in which there are about 3 Catholics left in the world.  This is the type of thinking that leads to home-aloneism.  I kindof called +Sanborn out on this one time.  He said that he was in communion with Bishop Kelly.  But Bishop Kelly was in communion with Bishop Mendez, and Bishop Mendez in communion with the Novus Ordo.  So I decided that I could not be in communion with +Sanborn.  Then it occurred to me that it's preposterous that I was pretty much the only Catholic left in the world, and that realization is what brought me back from the precipice of dogmatic sedevacantism.  Another consideration that helped me back away from sedevacantism is the notion that there are degrees of error that fall short of heresy in the strict sense, and that these errors do not make someone a heretic.  Sedevacantists like to shoot from the hip in accusing everyone of heresy, but the bar for heresy is rather high.  Then, the fact that people profess the Catholic faith means something, so it's a gross oversimplification to equate Novus Ordo Catholics with, say, the Orthodox schismatics ... which the dogmatic sedevacantists routinely do.  Finally, the argument that R&R "sift" the Magisterium based on private judgment also applies to sedevacantists, who sift Popes based on their private judgment.  So there were some huge gaps in the dogmatic sedevacantist position.

Father Cekada, who associates with Bishop Sanborn, once wrote an article about how the SSPV treats schism and heresy as if they were contagious or the "cooties".  So I'm not sure why the two haven't had a more serious disagreement.

This is the only thing I see mentioned regarding una cuм masses in its Theological Directory on the RCI website:

Finally I hold that the traditional Latin Mass which is offered together with (una cuм) the Novus Ordo hierarchy is objectively sacrilegious. Consequently I affirm that active participation in Masses or services in which the name of a Novus Ordo hierarch is mentioned is objectively a mortal sin.

I see nothing that states that Catholics can not attend masses of (or even go to confession with) those sedevacantist priests who believe otherwise.  I am going to give Fr Desposito the benefit of the doubt here.

I'm only reporting what he said to me. 

Jeremy,
Were these comments by Fr Desposito solely about the una cuм issue?  Or were you discussing a number of things?  Because I tend to think that he was not just speaking on the una cuм issue here.

It was all about the una cuм issue. 

.
Sanborn doesn't think the CMRI should exist and he doesn't let his seminarians attend mass with their families if that means going to a CMRI chapel.  What is surprising about also issuing a moratorium on Fr. Ringrose?

Yes, I am aware of issues between Bishop Sanborn and the CMRI, but it wasn't about the una cuм issue.  That's why I was questioning jeremy's understanding/relaying of Father Desposito's comments.