They do truly do think that. They don't mean that all are Derek Chauvin, but they mean that all circled the wagons around Chauvin. It was an obviously indefensible action, and he was still defended by his department, his union, and police around the country.

It was equally clear that he expected that from them, and that this was not his first case of abuse. This was merely the time when it became incontrovertible, because somebody had a camera and was willing to take the risk of filming it.

If they aren't going to stand up against abuse even when it's literally a murder caught on video, there's no way to avoid the conclusion that there are indeed countless other abuse cases that have been covered up, with nobody willing to stand up to it. It doesn't show the extent of the abuse but it shows the extent of police willing to cover up and excuse that abuse: universal. That makes All of them Bastards.