I would think there had to have been Japanese guys that went there and trained, given the way Japanese wrestlers tend to do that (make pilgrimages to the places where people they revere trained). I know at the very least Tatsumi Fujinami famously went down there, but I wouldn't think he would've been the only one.
Anyway, of course, Robinson's story of Kimura going to the Snake Pit and getting smashed isn't something I take too seriously. This is Kimura we're talking about. But, given that Karl and Kimura were friends, I just wouldn't be surprised that two friends that like grappling might at some point end up at a place like the Snake Pit. But who knows, maybe they never got on the mat. As I recall, Kimura did praise Gotch's newazza. But I mean, I have good friends that grapple that I've never rolled with, crazier things have happened.
But of course I'm sure you're right about the myths. Again, the whole idea of Kimura being mediocre at the Snake Pit is obviously just not very credible.