That is a valid theory, however I don't think we should label this place as the source of them all just yet. Indeed, their number is numerous here, and they have integrated somewhat into society, but that is just the normal adaptation of humans when forced to interact with something like that.

[Yamato thinks]

It is a situation where observation will be best, I believe. Of course, there is the matter of what will be done if this place is determined to be the source. Powers beyond are own are at work here, so it will not be as simple as merely containing them here. Demons are able to slip between cracks of reality because their forms are so undefined, unlike humans who are very anchored down and solid.