True, but I suspect that the Plasma Siege Cannon for champions and modular fortresses is based on the Fallen Spire Plasma Siege Cannon, which also does not fire on small vessels (tested the Mark I Spire Cruiser-mounted version against a combined group of about Bombers, Missile Frigates, Zenith Paralyzers, and maybe a few other things; I don't have a current game with higher-mark plasma siege cannons to play with, but I suspect they have the same limitations). Since Fallen Spire games have plenty of big things for a weapon like this to shoot at, it isn't really a weakness for the Fallen Spire version. For the champion and modular fortress, it makes the Plasma Siege Cannon a bit more of a specialist weapon. And the Fallen Spire version of the Plasma Siege Cannon is old enough to have been based on the pre-5.024 version of the Plasma Siege Starship.
So yes, I'd say that this particular module was drafted long before the first beta of Ancient Shadows came out. The hull bonuses, attack range, and ammunition types are identical, and the reload speed is the same for Spire Cities, Spire Cruisers, and Champions; the only difference between the modules is the base damage (the Spire Cruiser Plasma Siege Module Mark I is equivalent to a Mark V champion version, and slightly worse than a Mark I Modular Fortress version, if normal mark scaling is used for champions) and the cost, and the Modular Fortress version fires four times more quickly. All of them even have the relatively useless 10X bonus to turret hulls (currently useless for the player, anyways; not something I want in the AI's hands).