It‘s not just about the data—which is bad enough but as you said they could just write a crawler to get at it. The question is why would they want to federate and why now? Meta being Meta the most likely reasons are terrible for the fediverse and it reminds me very much of Google and xmpp. I saw a really good writeup on this yesterday: https://ploum.net/2023-06-23-how-to-kill-decentralised-networks.html