They are just silly, I have no idea how anyone can believe the entire Church has fallen except for their sect of 60 people.
Also this is my favorite thing on Sedes
http://www.papastronsay.com/resources/sedevacantism/index.php
>>815256
All the partial communion meme is that people who are baptized have some sort of relationship with the Church that non-baptized people don't.
It's used to confuse people quite often sure but it hardly is an even slight argument against the Church in any way. I don't know anyone serious who would say other religions are in any sort of communion with the Church though, there are probably some goofy people with some weird theology about how worshiping any monothiestic God is in some sense being in communion with the Church but it's so utterly detached from how anyone actually uses the word I don't see why anyone should even care.
Also the idea of a random lay person totally grasping the problems in the Church is the actual problem, all the authority is above us, and God knows far better then any of us. It just doesn't make sense for us to be able to grasp everything and then act on it. It's reflected in their idea they can hear some youtube video and read some old documents they don't understand then pronounce the pope isn't actually the Pope, we can't understand everything but we can understand what we ought to do and that's what God gives us.
Reminds me of this from Chesterton (the whole section on the madman in general but this line in particular)
Poetry is sane because it floats easily in an infinite sea; reason seeks to cross the infinite sea, and so make it finite. The result is mental exhaustion, like the physical exhaustion of Mr. Holbein. To accept everything is an exercise, to understand everything a strain. The poet only desires exaltation and expansion, a world to stretch himself in. The poet only asks to get his head into the heavens. It is the logician who seeks to get the heavens into his head. And it is his head that splits.