By an "alarming amount" don't you actually mean a fringe minority largely comprised of ignorant dips that barely bothered to expand their worldly knowledge beyond Japan?
Secondly, what OP means is that (and this is something i've noticed as well) Japanese tend to be very easily influenced when it comes to their interactions with foreign acquaintances so their opinions tend to be colored by those interactions as well. I find this to be true mainly in those who study English or have interest in foreign cultures; almost as if trying to appeal to those cultures albeit picking up some pretty awful cultural biases while not fully understanding what they're parroting. Likely this guy is close friends with some alt-right sperg who's presenting his opinions as fact to Nobita, who hardly has any outside perspective nor can he grasp the cultural nuances well despite his attempts. This can actually be applied to anyone raised in one culture and attempting to immerse themselves in another; again, in an attempt to appeal or ally themselves more closely with the culture they admire they tend to pick up certain points; i've seen quite a few weebs picking up this attitude and going the "I hate Koreans/Chinese because (insert BS here)" route because of shit they hear from their Japanese acquaintances.
Honestly I think it's pathetic for people to just parrot shit and not use their own judgment and research to come to such conclusions, especially when they're adults with supposedly fully developed brains, but it seems to be a flaw in human social functioning or whatever.