I recently found this game on some reccomendations from Reditt, and am having a good time with it. I'm only on my second playthrough & have not yet purchased the DLC, as I wanted to try it out first, but I have just run into the Tinkers as one of the AI players (they are not available as a selectable race for a new game). I also have the Asteroid base in my tech tree, which someone said is part of the DLC.

Is this expected, or a bug?

It's expected. Tinkers empires became an official part of the main game when the DLC launched. Buying the DLC gets you the ability to play as this new faction, but they're meant to be a part of the game regardless.

Thanks for the confirmation.