I have the same question

Didn't it mean that the item was in the pawns inventory but cannot be used during combat? So the pawn is basically carrying it around.

As far as I can tell, it means that the pawn has an item in their inventory that is a weapon, but it isn't configured as a "sidearm" (or primary weapon) as far as this mod is concerned. Usually this happens when a pawn is gathering up, for example, weapons and armor and other dropped items after a raid (via Pick Up and Haul), but I've also found occasionally that weapons actually equipped as sidearms randomly flag with the diagonal stripes (though the pawns don't seem to seek to auto-unload those like they do the ones they picked up via PUaH). Dropping the item and picking it back up against as a sidearm usually fixes the issue for me.

I've not found any real side-effects of a weapon being flagged that way, tbh. But it might be that pawns won't auto-swap to weapons flagged as such, even in situations where they otherwise would (ex. a melee weapon when they get attacked in melee).