Agreed on your corrections, except that when I said dart gun I didn't mean the blowdart kind, I meant the modern kind that look like actual guns with triggers (like how zoologists sedate wild animals sometimes). Still silly to put a camera on one? I guess, but if that's what you have as a "gun" then it makes about as much sense as us putting them on our guns, maybe more because dart guns don't have muzzle flashes and smoke.

are modern assault rifles really that much louder than muskets?

Modern assault rifles are somewhat louder (and I think higher pitch?) than muskets, so they are more prone to causing hearing damage than muskets were and I could definitely see that being a shock, but again look at how abstract his confusion is. If that dude was intimately familiar with muskets like we'd expect an interplanetary invader to be if their army used muskets, then he might find the compactness or any of several other features about the gun shocking, but they don't even identify it as being a type of gun, which a musketeer would be able to do even if they could only fathom its operation as being a matter of magic, because it's still sort of the same shape, with a barrel and trigger and so on, making a bang, and shooting a bullet.