Yeah it was bench seating so one guy had the reins and the other had a shotgun. Hence the name.

The apocryphal story is actually kind of interesting.

Roads and right of way established during the pre-firearm era were that you'd ride on the left, with people going the opposite way on your right. This was so you could use your dominant hand (usually your right) to use a sword to defend yourself.

Roads after firearms were available often established right of way with riding on the right, with oncoming traffic on the left. This is because when you shoulder a firearm on your right shoulder it's easier to aim left.

Stagecoach drivers would sit in the left seat, with the extra person sitting on the right, holding a shotgun, hence the colloquial term for the front passenger seat.

I have no idea how true this is, but it makes for an interesting story.

I'll try and explain, but let me know if you don't follow. In the US it's common to claim the front passenger seat by saying "I call shotgun!" or simply "Shotgun!" The commenter is playing on a now common refrain where Americans use firearms and terminology to describe basic things. As far as I can tell, it's true. For example: caulk gun, staple gun, nail gun, glue gun, tattoo gun, finger guns, ot phrases like "I'll think about it before I pull the trigger on it." Or "Shoot me your email and I'll get you those photos."

I don't know how prolific this type of thing is in other countries though, so I can only assume we Americans arr outliers due to how deeply ingrained guns are in our culture. Hope this clarifies things a bit, let me know if not.

TLDR: Americans describing so many things: "So imagine a gun, but..."

They're saying, no, it's not common for other cultures to call it a gun thing. But in a humorous way, by drawing attention to the absurdity of the question.