Hi, thank you for grabbing our game!
Yes, this is intentionally controller-only, to have the gameplay authentic PS1 experience. (I even bought a new PS4 controller to get it working best with it, cause someone in our Team only uses that to play. But any Xbox compatible controller will also work).
I was considering to add alternative inputs, yet 'cause of the skill it takes to fly, aim and shoot down enemies, it would have been a rather subpar experience.