Actually, the article says that for a pope to be deposed, they either need to separate themselves from the church ("walks away" as you say),
or the church itself has to judge them as a heretic. Their whole point is that, contrary to the claim of sedevacantism, individual Catholics don't get to just say "well, he sure seems like a heretic to me, so he's no pope!" Robert Bellarmine's statements regarding whether a heretical pope can be deposed, and how to do such a thing, have been used by sedevacantists focusing in on the fact he indicates he believes such a pope would be deposed by God rather than the church, and thus they claim that means the heretical pope loses office without any action needed by the church, which is required for the whole idea that individual Catholics can simply judge the matter for themselves to work.
The argument of the article is that this is a misunderstanding of what Bellarmine was saying, and that Bellarmine affirms that no pope loses office for heresy until the pope formally and explicitly quits the church or church deposes them for it via a council of bishops (it notes that "deposing" is perhaps not an entirely accurate term, as it is rather the church declaring what had already happened--but in terms of practicality, it means the pope continues to exercise his office until the declaration is made).
Some portions of it are a bit complicated, but that's because it's going through the intricacies of the theory of Bellarmine (and others) regarding how the process works on a theological level, i.e. how a pope gets deposed if a council supposedly can't judge them, and things do get complicated there. But its
bottom line is that Bellarmine (and various others) that sedevacantists claim affirm that a pope just automatically loses his office even without a judgment of the church is not teaching that.
That said, I pointed to that article because while it was complex, it seemed to be specifically addressing the arguments of some people you had referred to. If you want something by the same author that makes the same general points in a simpler manner (without addressing anyone specifically), this one also works even if it is a little out of date:
A site about a new book responding to the arguments of sedevacantists as well as a course on ecclesiology.
www.trueorfalsepope.com