How does he know? Is there any evidence for this? Were women or blacks 'barred'? Because if so, I would expect someone to sue and win easily.
The rest of the tweets are a stream of similar and all-unsubstantiated accusations. But we can't dispute any of what he said, because he made his accusations without enough specificity for them to be checked in any way. He chose not to name the 2010 conference.
There's nothing trifling about such accusations. Just because nobody is in a position to respond doesn't mean you're not hurting people. Whether the accusations are made against individuals, specific groups, or a whole ethnic group. If you're gonna slag someone you should have evidence, and you should be specific.
In any case, he chose a good strategy to maximize his personal outcomes. He'll benefit greatly from this. D on fairness and principle, A on strategy.