Is it really fair to call Facebook just one bad actor? It's one of the largest corporations in the world, has some of the largest social media and messaging platforms out there. In terms of resources, there are very few companies, let alone individuals or groups, that can compete with Facebook.
If you look at it in these terms, you understand that Facebook has an interest in making sure that ActivityPub doesn't too large without Facebook having a say in it. If it could control the whole internet, I'm sure it would. So, no, I don't agree with your framing of the issue.
But that wasn't my point. It's not that I think that Facebook or Google cannot scrape Fediverse platforms/instances, it's that even if they do, they cannot serve targeted ads based on our activity here.
We have different definitions for privacy. Since I'm active here, it should be clear that to me private doesn't mean hidden. I like how the EFF put it, in their article on the Fediverse: