It's generally true, but the reasons why are a bit complicated.
First and foremost, in the words of a friend, East Asians and people of East Asian descent (eg. Japanese-Americans) visiting Japan are "stealth gaijin": it's not immediately obvious that they're foreigners. And in general, the Japanese tend to be extremely hospitable to people they consider "guests", but keep their distance from people they do not know.
So consider a hapless visitor to Japan, scratching their head in dismay while staring at a map of train line spaghetti in Shinjuku station. If the visitor is a blond long-nose like myself, it'll be pretty obvious that they're a tourist and that they're lost, and an adventurous kind soul is likely to stop and offer to help. However, if the visitor happens look Japanese at first glance, he'll be assumed to be Japanese, and left to his own devices — because surely he can (for example) go to the information counter or police box if he's genuinely having trouble figuring out where to go.
Also, if said Japanese-looking visitor actively asks for help, especially if he does so in (bad) Japanese, the initial false impression of being Japanese may be so strong that some people will continue to assume that he's Japanese, but eg. mentally handicapped or simply rude, because he's not behaving like a "proper" Japanese — who, again, would likely not be troubling strangers, certainly not without the elaborate apologies demanded in this situation, but would go to the information counter — and brusquely refuse to help.
The above, though, would only happen if the person presents as a pitch-perfect Japanese, and given a reason to look twice most Japanese are actually pretty good at spotting Chinese and Koreans, not even so much by appearance as by makeup, clothing, etc. (It's still an art, not a science; I had a friend who always passed for a local in all three countries.) So the final possibility is that they realize the person is non-Japanese, but are still reluctant to help because they're afraid of scams: non-Japanese Asians are somewhat unfairly associated with crime in Japan, and they may fear that the apparent hapless tourist is (say) trying to distract them while their pickpocket buddy slits their purse.