Well, what it really means is you can't Mod Unique Weapons. Which some of the Pre-Order SWAG is just that, Unique and hence un-moddable.
I am surprised the game even let you put the silencer on the gun. Must be a bug that allows that. As for the weapon screwing up because you did something you are not supposed to do, I would not call that a bug.
Aye, there's the rub.
The problem is, that verbiage
isn't present in any official capacity, anywhere; there is no "weapon screwing up because you did something you are not supposed to do". It's simply a consequence of using an existing game mechanic available for the Weathered 10mm pistol. It almost seems as if you're being intentionally obtuse...I don't know what you're hoping to accomplish.
Split hairs all you'd like, this is a defect. Like any other defect it needs to be identified, reported, and most importantly, fixed.