Were they using Twitter to provide exclusive updates not available anywhere else?

My impression from the post is that they are publishing the exact same updates in multiple locations, including mastodon at https://framapiaf.org/@debian ...so just because they were publishing in that one extra site to make it accessible to a particular subset of people does not mean all other people were being shut off from receiving updates.

However, I do agree with the move, but only because Debian being a FOSS initiative should stay away from proprietary platforms and promote FOSS.