I think the important bit is that it's relatively hard to argue that "how to scale up message passing infrastructure and process metadata like hashtags" is a Twitter trade secret rather than a general tech skillset useful across FAANG companies. I suspect Meta's implementation will be quite different from Twitter's under the hood, and rely quite a lot on Meta tech platforms.
If Meta was incubating a rocket ship division, it might be a little bit easier to demonstrate that former SpaceX employees were making non-obvious design choices based on SpaceX research and undisclosed features of SpaceX tech
Probably won't help that a lot of the more Twitter-specific knowledge around content recommendations and warning algorithms and scaling up moderation processes is stuff Elon's spent his time trashing Twitter's original implementations of anyway...
One could imagine, that Instagram, on which they based it on and apparently reused most of the backend dealt with some of those problems before. And I've heard that Meta had some other social networks that scaled in the past.
