He refused to sign an NDA, so he explains briefly in the Twitter thread, and we'll probably hear more later.
In short: Sudden firing (as in, locked out of Slack and email while at work while being informed of being fired) for not "meeting goals", where goals were not clearly defined.
Looks like standard corporate damage control NDA where you are not allowed to discuss the reason or terms of your firing. Usually illegally demanded to get severance pay.
I was laid off earlier this year and was locked out of my Google account and Slack and everything else company related immediately after my meeting with HR and my manager ended. It was shocking how quick they were about it.
I kind of wonder if Google Workspace has a "Fire Employee" button for administrators. I think it probably does.