Is there some gnome magic we don't know about?

He seems acrobatic - perhaps the swamp can also let people in, but it won't let people out.

He seemed to get out easily enough after you restored his humor. At least, he never went back to the inn and wasn't seen anywhere else.

But yeah, he had some acrobatic skill, so maybe he was able to jump his way across the swamp, both coming in and going out.