I get that, and I agree with it in general, but there's literally no company on earth that would approach open source developers with the intent to pay them to work on a closed source product, or to buy out their open source work without having an NDA in place. Hell, even if Meta just wants to pay them to do open source work to support the community, there will still likely be an NDA covering what they can say to the public about the arrangement or anything they learn from having access to internal systems.
It's like saying "Meta has security guards at the doors to their datacenters! They must be doing something illegal in there!"
Meta is evil and is very likely doing something bad with these developers, but the NDA isn't the smoking gun evidence of evil... It's Meta's history in general
I mean, the real answer is that most open source developers aren't here for freedom at any cost. They're here like a startup... Waiting to be acquired for big bucks. Open source doesn't pay bills, and if a megacorp pulls up in a Brinks truck full of cash, I wouldn't be surprised if 80% of open source projects sell