There certainly are similarities with the Old Catholics in their view about the Church of Rome.
And there are no similarities between what St. Robert and what they are saying about a heretical pope.
They ignore St. Robert's teaching about the local Church of Rome. He wrote a whole chapter on it but it is conveniently ignored by the sedes.
I am curious about the point you try to make here. It is my understanding that in the mentioned chapter St. Bellarmine is defending the proposition that the local Church is indefectible precisely
because the legitimate sucessor of St. Peter lives there and will not transfer to any other episcopate but Rome. We will never have a pope who is bishop of Constantinople, for example, but only of Bishop of Rome.
If this is so, then I don't see how this argument can be made against sedevacantism, because the promise of indestructibility of the local Church of Rome would be compromised when there is NO legitimate Vicar of Christ living there. During an interregnum, there is no Roman