It’s actually Occum’s razor that’s being applied here; the explanation that makes the least amount of assumptions is the most likely to be correct, You’re making the assumption despite a complete lack of any supporting evidence to the contrary that the Vindicar is a viable weapon’s platform outside of the argus theatre.
And the simple fact is that canonically the vindicar has never engaged non-legion forces so we can reasonably conclude that it isn’t a viable weapon; it’s weapons either aren’t effective against non-fel targets or based on how we had to dump endless amounts of pristine argunite into the thing that it’s way too much of a power hog.