No, it's not.  1983 Code of Canon Law says it's allowed for a Catholic to attend valid Masses of non-Catholics provided there's some just cause ... including even a full-blown schismatic Orthodox one ... and that these even meet the Sunday obligation.  So if these guys are your hierarchy, then they said it was OK.

Alright lemme explain what my convictions are on this matter.  Note that I'm not saying *anyone else* has to do what I think is right, or that I'm necessarily fully consistent, just this is basically my best judgment right now.

I'd probably get broadly categorized as "Loosely Neo-SSPX" by people on here, which I still think is distinguishable from FSSP but we've talked about that before.  

I have an SSPX 30 minutes away from me.  And I have an SSPV an hour away from me.  Having a completely acceptable SSPX (I don't have an issue with the NO rites of ordination, but he's traditionally ordained as well if that were a concern), *I personally* feel that to *regularly* choose to attend SSPV when I have a perfectly acceptable SSPX option that's closer would be to effectively reject communion with Francis.  

And since I believe (while I wouldn't say I have certainty of faith, I believe I'm obliged to give the benefit of the doubt so to speak) that Francis does indeed hold the papal office, I believe I'm obliged to, if possible, pray for him daily and attend a mass that prays for him.

I don't know if the '83 code defines just cause, but like, if I lived in a place where my only options were Sede mass or NO mass, I would consider that a just cause to attend the Sede mass  That's because I believe objectively speaking the NO mass has been protestantized and thus is best avoided.  In that situation I'd kinda be like... yeah there's no intent to break communion with Francis here, I'd rather have an SSPX mass, but I still think going to the Sede mass is better than not attending mass at all (and I think due to the great confusion that we're currently in, I do still view the Sede chapel as a Catholic Church, so I'm not even gonna address the Orthodoxy thing as that adds an additional layer of complexity to things.)

Similarly, if my only options were indult or NO, I understand why other people's convictions are different, but I would in that situation go to the FSSP parish.  However, I think to *regularly* attend the FSSP parish when one has an SSPX parish close by (like less than an hour I think) one implicitly affirms the legitimacy of the NO, 'cause its like... otherwise why wouldn't you attend an SSPX over an FSSP?

I think occasional visits to a place one doesn't fully agree with (And again I'm talking about Catholic parishes, I'm not talking about non Catholic places of worship, I'm talking like an SSPX person visiting an indult or sede chapel) can be OK.  I don't think "I go to the FSSP every now and then for fellowship" says you agree with them in the same way that "I go there for mass every week instead of the SSPX does."

So yeah, basically, I wouldn't go to a Sede parish regularly if I had some other option (non NO.)  I'd honestly rather pick a Byzantine or Indult chapel under most conditions with where I'm at right now, barring Corona mandates which are honestly kind of my #1 issue with churches at this point (I won't wear masks in church period as I think its disrespectful).

All that said, I certainly have no *authority* to tell anyone *else* they can't attend a Sede chapel for whatever reasons seem good to them.  

The 55 Holy Week is what would be used for the vigil