There is no expectation that anything said on social media will remain private - zero - and anyone that thinks differently is just lying to themselves and everyone else.
Bluesky had a similar freakout when HuggingFace packaged up a snapshot of the firehose and this discussion was had ad infinitum - in Bluesky's face every post is explicitly public -- but a very vocal minority of users still felt they had been wronged because no one asked if they were OK with being a part of a research project. There's definitely a gap between what users /should/ be aware of and what impression they actually get using a service, and you have to keep in mind most people are not techies that have spent years of their life wrestling with issues of privacy and data ownership, so announcements like this can come as a surprise, the first time they considered that what they said in one context can be moved to another.
Plus, even if the server is in Discovery (and thus really publicly advertised), they’re still mostly sorted into 10+ rooms. Just because a library is public, doesn’t mean I should expect Study Room B will be recorded by John Jameson and entered into a public dataset.