I'm afraid you've been misinformed. I think you'd find the breach with the IS took place over the issue of the Portugese revolution, or at least that was the specific catalyst. That's how he seems to remember it in an old LRB article, and it's how Alex Callinicos remembers it. By the time he had moved to the United States, Callinicos' judgment is that he was a 'left social democrat'. The fact that he continued to speak at SWP meetings is not relevant. Hitchens was appearing at SWP meetings and speaking at Bookmarks right up until he capsized post-9/11.