A license that requires source. And since then there have been many different licenses, all with the same requirement. Giving someone a binary for free and saying they’re allowed to edit the hex codes and redistribute it doesn’t mean it’s open source. A license to use and modify is necessary but not sufficient for something to be open source. You need to provide the source.
I don’t trust it because there’s no believable plan to make it commercially viable, so it’s just going to end up defunct or enshittified. Mastodon is up front, it’s a volunteer service that you can either pay for or roll the dice on the instance staying up. And there’s a built-in way to move on when one goes down.
BlueSky is a B-corp, which theoretically means they can say their mission takes priority if sued by an investor in court, but doesn’t in any way require them to make it the primary goal, and the reality of funding and money and investors means that’s almost certainly not going to happen.