Discord has been available for training data for at least some months. I used duckduckgo to search information on my gamertag and whichever AI they were using at the time came up with results that would have only been available had it used Discord's data.

That's my read on it, they might use this to treat these messages as public information to be packaged and resold like how Reddit can rather than it being a place where people have an expectation of privacy.