Of course, secret apostates/etc. would remain members of the Church. These are all external forum considerations.
I've heard this mentioned before, possibly in relation to Bellarmino.
I don't understand how this would work.
I suppose he theorised this in order for all the jurisdictional issues to make sense in such a case, for a cleric etc.
But formal heresy, "in the internal forum", exists and has effect regardless of we the mortal people being able to see it.
It's one thing the interior, or divine, realm, supernatural in essence and another the visible and manifest (external forum).
What happens in the case of a formal heretic, non manifest, as to its jurisdiction is in my opinion the Church supplying it automatically were this case to arise, as to conform the Divine with the Apparent, for the benefit of the faithful.
He would still de facto be valid and his acts licit, but as a private person, he would be outside and a non member.
Thinking back to the Schismatic "orthodox" example from the Sedevacantism thread, that's exactly what happened and how it could have possibly been licit for Catholics to receive Sacraments from them.
The Church (this time consciously and actively by means of Papal pronouncement) supplying jurisdiction on a case to case basis, to clerics who were non members and outside the Church.