Well that bio surely matches with the whole thing. The fact that she's an acquaintance of Xia-san really makes this seem plausible. Now if only we get some info if she actually is out in the open anywhere at all...
The quote in the OP is from the secrets guide on GameFAQs by the way.
Giorgio wrote: By the way, should be not: "hidden tutorial on reading house signs to find Yamamoto"?
Well, not how I meant it. You see, during that part the player has learned that Yamagishi saw the black car and thus has to search for his house. It's the first thing that you have to do for yourself in the game basically because everything before that were unavoidable, triggered scenes. To me the search for the Yamamoto's house always seemed like some sort of tutorial on how to find out who lives in those houses and that you can lock on to the signs to read them closely. In the Japanese version this is even more apparent because it doesn't have the yellow subtitle of what store or house you're looking at that even works from a distance.
But well, that's the way I kinda see it.