The second point only works once they meaningfully federate and stay meaningfully federated.

But more importantly:

1. There is probably no Twitter IP at issue. There could be some patents, hard to guess, but I imagine Meta and Twitter have a cross license, or at least a detente because they could sue each other so hard that only the lawyers would win.

Edit: Oh, Twitter is accusing Facebook of misappropriating trade secrets. It's theoretically possible that one or two laid off twitter employees reused some trade secret information, but... I feel like this is a fishing expedition, Twitter doesn't actually have any suspicion that Facebook did that, they just wanna be dicks about it.

1. They can't be monopolizing this space while Twitter still has almost all the market share. They could be accused of attempting to monopolize if they did things like predatory pricing, but that's a hard case to make, and even if they do gain market share, at this point, it's going to be because of Twitter actively ruining its own product and throwing its large positive network effect advantage right in the trash. Nobody could possibly blame Facebook for that, Twitter would never win, even in a country that did enforce antitrust laws against tech companies.