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.
You appear obsessed with this Church of Rome issue. But has this Church of Rome retained the faith any more or less than the broader Conciliar Church? No. So it's a moot point ... even though you think yourself quite clever. Yes, some critics of sedevacantism make the so-called
Ecclesiavacantist argument, and they have a point. But that's where the CT and so-called sedeprivationism fit in.