I'm reading through the Twitter thread, but I can't decipher the exact issue(s). Did the organizers select a lower ratio of diverse speakers than applicants? Were there very few diverse applicants and the organizers decided none of them qualified?
Twitter is never a great forum to discuss issues that need context. The two blog posts lay things out more clearly.
Regardless, the conference organizers are at least partially responsible for lack of diverse applicants if their speakers were able to find candidates on short notice.