Libel would require the allegations be, at the very least, false, and not an opinion. It appears all factual allegations here are true (he presented the talk, he used bits from another talk, he called a speaker wrong), and everything else is an opinion based on those facts. So it's not libel.

And any improper employment termination suit would be against the employer, not a conference organizer, because in a very direct sense, it's your employer that chooses whether to fire you or not.