I consider the Dimond Brothers and other extremists to be schismatic, since they take a chainsaw to the Mystical Body of Christ by excommunicating all and sundry.
I was in the Dimond camp for a long time, and it's this that drove me away from them and made me realise they're nuts. Imagine a 60 year old man when Vatican II was in session. He had been a faithful Catholic all his life, going to Mass every Sunday and trying his best to live a virtuous life, and going to Confession whenever he faltered. Vatican II comes and goes and he's a bit surprised and confused about the Mass being in English now, but the Church does what the Church does and he has no reason to question it. This was before the internet, a random working man somewhere might never have even heard of the SSPX when it came around, let alone that the new Sacrament of Ordination is somehow invalid due to a change in the prayers said. That's far too deep and esoteric for a random layman to know about, especially when he has no internet. So he continues on, living his life well and devout, until one day the local parish Priest changes and the new one is a young man who was recently ordained under the new Rite. The man keeps going to Mass and continues going to Confession under this new Priest. And eventually a years later the man gets ill and passes on as all men eventually do, and receives his Last Rites(but under the new form).
According to the Dimonds, any man like that one in the story was damned to Hell if he committed a single mortal sin in all the years after the young Priest took over the Parish. All the Confessions and the Last Rites were apparently invalid due to the new rite of Ordination and the man was damned despite having no way of knowing that. And even if the man didn't commit a single mortal sin in all those years, he'd STILL probably be damned according to the Dimonds because he went to a Novus Ordo mass and believed in the "Vatican 2" religion rather than the Catholic faith, again despite the fact he'd have no way of knowing any of that. And there were millions and millions of Catholics who died in the situation of that hypothetical man - and according to the Dimonds, and I've asked them this to confirm, ALL those Catholics were damned through no fault of their own(although the Dimonds insist it is their fault because "the information was out there!" despite the fact that not even the Dimonds themselves were sedevacantists until the 90s).
Even if the Dimonds were right about sedevacantism, all those Sacraments being invalid, etc.(which is a HUGE if), the assertion that tens of millions of Catholics could be damned through no fault of their own like that is an absolutely ridiculous notion. It's exactly like what you said - they take a chainsaw to the Mystical Body of Christ and cut off basically everyone who isn't a member of their monastery or a user of their website.