That was NOT a secret. I didn't find it as a kid but I've seen way to many people say they did after that story cam out.>rainbow coins are worth 25>speedrunners think the game has 975 banana coins total>speedrunners are too stupid to stop and think, gee, maybe's there's a missing rainbow coin
And the coin was not THAT hidden. It's in a mound in the grass in a place you frequently walk back and forth over. People found it, but no one thought to notify the wider speedrunning community.
It's like how there's a Shock Jump Disc behind the entrance to Click Clock Wood, which you can tell is there by the metalic sound you footsteps make when you walk over it. It isn't impossible to find, and some people found it. But it's obscure, so it's not well documented.