Not a poor assumption at all. With under a year of Japanese study and in the context of playing tourist for a few weeks, there's no way you had enough exposure to Japanese people and life to issue a blanket statement like: "Japanese are incapable of understanding a gaijin doesn't always need to be talked to as if they're five years old, they always have been and always will be."

I can certainly understand the genesis of such an opinion, and there's a modicum of truth and accuracy to it. Not enough to save it from being hyperbole, unfortunately.

People here don't talk to me like I'm five years old. Not to say that they never have, but it has been a while.