Interesting thought. Indeed, the person might think it’s some sort of exotic bolt thrower.

Pliny, as an extremely well-educated man, actually wrote a bit on chemical and incendiary weapons at the time. So while the notion of the chemicals being a propellant would be foreign to him, a warning shot from the smoke-and-thunder tube would probably give him a good basic idea of what he was facing - a machine that inflicted some form of harm via a projectile accompanied by fire and chemicals.