This literally happened when I was visiting Japan and your belief is invalid. My girlfriend fell down the stairs in the Tokyo train station during the rush hour commute and many strangers were concerned for her well being (to the point we had to decline several times being taken to a hospital).
i was offered help multiple times when standing alone by myself when waiting for something, or someone. i'm an asian male, pretty much indistinguishable from any random japanese guy.
i've been a lot of places and i have a hard time believing that you could fall down in the middle street anywhere on earth and nobody will help you, including new york, shanghai, other megacities.
excepting maybe some extreme examples like communistic societies at the height of the cold war, but that's it.
If a random guy collapses on the street nobody would care.