You're not alone. I certainly expect Webb to cast only pro-gun votes in the Senate, including to rid DC of their gun ban. I don't expect anybody to make needlessly self-incriminating statements. There is no way that I can imagine that any statement by Webb could establish the innocence of the aide.
How could Webb possibly state that the aide didn't know the gun was in the bag? Webb wasn't present when the aide took possession of the bag. Only the aide can state whether he knew the gun was in the bag.
Maybe he meant that he (Webb) didn't tell his aide that.
IMO it's plausible that there were a few bags in the back seat, the aide just scooped them up to bring them to office.
Who knows.