I’ve heard it before. It doesn’t pass the kook test, which is why I’ve never looked into it. The proponents of this theory claim that he was drugged and threatened for 10 years (if I recall correctly,) which is why he never publicly claimed the papacy for himself, nor hinted that John XXIII or Paul VI were antipopes.
It strains credulity to think that such a conspiracy could be carried out for such a long time by so many cardinals and bishops. He was close with Archbishop Lefebvre, Cardinal Ottaviani and other “conservative” Council Fathers. He probably received votes from Cdl. Ottaviani and others who preferred Siri to Cdl. Roncalli (John XXIII) at the conclave. So its inconceivable to me that they could either be part of such a conspiracy or ignorant of it for all those years.