It doesn't much matter.  Think of it this way -- if it were true, and he were elected Pope but then was forced to resign, how would he have called a college of cardinals together to elect a successor?  

The Siri theorists believe he was not only Pope, but that he started his own underground line of Popes.  But at the same time, the whole reasoning behind the Siri thesis is that he was pressured not to say he was Pope, and that he was followed and kept under surveillance during his whole life to make sure he didn't spill the beans.  

So let me get this straight -- he was kept under surveillance so intently that he could barely breathe, he couldn't speak about his true status as Pope.  Yet somehow, despite this 24/7 surveillance, despite the bodyguards who would kill him if he so much as breathed a word about his true status, he was able to appoint a bunch of cardinals who would elect his successor?

Doesn't really work, does it?  David Hobson, main proponent of the Siri thesis, once wrote to me something about how he can't tell me now how Siri appointed a successor but all will be revealed, etc.  A scam may be involved whereby someone will emerge out of the blue with the fabled "successor to Siri," who would really be an actor or vagrant priest or something like that.  But I think the Siri thesis is not really taking off like Hobson had hoped, and it's petering out more and more by the day.

Whatever the problems with Gregory XVII, there are far more with any other scenario. Over 30 yrs he would have had the opportunity to appoint a curia one way or another.

Can it, Augustine.

Not all proponents of the Siri theory believe that he appointed a successor.  There's no credible evidence for that.

I do believe that he was elected and forced to step down.

Yeah, there are lots of questions regarding the details.  We don't know whether everyone there knew he stepped down under duress.  It's possible that the threats were communicated privately and that many or most of those voting in the conclave were not aware of why Siri stepped down.

If Siri was elected, accepted, and then resigned under duress, then the elections of J23, P6, and JP2 would have been invalid.  St. Francis had a prophecy about an "uncanonically elected" pope who would wreak havoc on the Church.

To me, the best evidence might be the FBI docs cited by Paul Williams.  It not only mentions that Siri was elected but that he had taken the name Gregory XVII.  That's some pretty specific information (the exact name) which also indicates that Siri had in fact accepted the election.

I do not believe that the last John of the GWS has been formally declared an anti-pope. My understanding is that he actually was the one GWS Pope who waxed heretical but that he recanted and it was forgotten.

There was a prev John 23( or 22?) as an anti-pope though.

Malachi Martin  wrote about this in Keys to this Blood. He said that Siri was put under duress in what he calls "the little brutality."

I think this is a very likely scenario, if he declined the papacy while under duress this woulld maKE HIM THE TRUE POPE. It would be virtually the same if they came in with strongman and just kicked out the old pope and put the new guy on.

I don't have the reference handy now but I believe he was selected again in 1964 and in 1978, according to Malachia Martin.