Perhaps the garden area wasn't meant to be there. Maybe the house was designed to be rectangular, and the hallway was going to lead to a room which didn't get built.
I dunno, I think it was hinted well enough that he was going to be like this. Even in the first chapter, we learn a lot about how the original Golden Man fights. He's silent, doesn't talk to or reason with villains, doesn't have a catchphrase or otherwise interact with his fans, doesn't care...
I think an important part of the ending is buried in the background of page 18. I think Maa-kun might be a member of Houjun's family (or dojo?). Houjun's home has the same wooden slat flooring/walls as the room Maa-kun is playing in, which suggests to me that those locations are the same building.
That last page is fake, for the record. The chapter ends on page 19 with Nagatoro calling Senpai gross. Looks like the scanlator's playing a bit of a prank.