Every regional unit takes bonus damage from the Spear Line, including the Dromon.
The only none mounted unit before the Dromon was the Eagle Warrior which functions as a cavalry replacement.
So yes, all regional units are cavalry.
The game could certainly use some none cavalry regional units (I complied some ideas that don’t include cavalry):
[Just for fun] Reginal units for hypothetical new civs and old civs
The Catapult Ship idea I had was actually added for the Romans.