This is my guess. He probably knows a sale is happening and doesn't want to provide value to the new owner. Probably because he has a competitive portfolio company or something.
That sounds petty and spiteful in a way I wouldn't expect from him, especially so publicly.
Also, as valuable as his presence is, I can't imagine a buyer of the size needed to buy Twitter would be like "we'll give you n Billion, but only if pmarca stays on point"
This could be a legitimate concern of his. But I would guess that even if he were to delete everything, Twitter would still have permanently archived undeleted copies (Facebook does that apparently).