I don't know enough about GC to argue about it, but the way I read his response (including the next paragraph which is not about multicore) he seems to claim that there is not a severe performance penalty for a C4 style collector, contrary to the email from Ian Lance Taylor, so I'm not sure he missed the point. He also has a graph showing supposedly how much it's not a severe performance penalty.

He also has like 5 more responses, not just that one.