@goarachnidlaser If you look closely, the door rules haven't changed, except maybe a bit of license at the very beginning.
Anyone can come into the room from the Japan side and the door will be open to Japan - until it's closed. The artistic license here is that the door should have only opened to Japan if the salesman was turning the knob from the outside, but hey, plot convenience.
The salesman left through the front door into the dungeon - exactly as expected.
Now watch as the salesman is talking to the monsters - the monsters decide to leave, and the salesman puts his hand on the monster's shoulder as they walk through the wall. That's also within the rules - as long as one person is allowed to pass, anyone touching them will pass as well.
Personally, I found this chapter to be hilarious. Pushy assholes like this guy exist, and this one refused to listen to anyone and ended up lost in a dungeon on another world. He got what was coming to him.