PG clarified (on the thread) that if he chose the other way (staying on YC) it would have been fine with them as well
If my boss told me he was "fine with me staying at my job", I'd take it to mean that they were fine with me staying at my job.
I don't think this press release (is it that?) does a very good job of dispelling the rumors that it wasn't entirely mutual, though. He ought to have said something like "delighted to have him stay".