While they do seem to be pro Ukraine, that's not their raison d'être.
They're just a national defense/defense industry shitpost community with what seems like a genuine appreciation for cool vehicles, aircraft, and other equipment, even if most of them would prefer that equipment was never used for its intended purpose. Credible defense was for serious military posting on Reddit, but I don't think there is a lemmy community for that.
Right now a lot of that equipment is being used in Ukraine so a lot of posts reference it.
Edit: Also, Russia is currently providing a lot of fodder for shitposts.