> He quit that group because he can't sign a statement that he is anti-racist, and supports diversity and inclusion?

Incorrect, this statement was not directed to members' conduct. The diversity statement was to pledge that members' research submissions are advancing anti-racism, equity, and inclusion. This would, for example, prohibit a psychologist studying something like memory retention. This has no reasonable link to advancing equal racial outcomes. How does measuring the amount of time it takes to memorize a paragraph advance racial justice? Thus such research would thus fail to live up to this pledge, and be ineligible for submission - if this pledge were actually enforced, that is.

Of course, I doubt the people making this pledge actually intend to have every piece of their research connected to an anti-racist goal. This is just performance and naval-gazing.