Yeah, that's true, but from the sound of things it took a while to actually set up the barrier to keep demons from crossing from one side to the other. Probably didn't have it fully up until a little into the Tokugawa era if my knowledge of those two eras can be trusted.

Until they set it up I always figured that demons had no problems crossing between worlds. But that's just judging from the stuff with Raizen and Yuusuke's other ancestress late in the series.