it is not a secret but very unexpected. when I heard of donkey kong, it was in context of donkey kong franchise which obviously feature Donkey Kong as main character just like mario is main character of mario games.

so technically first Donkey Kong game in Donkey Kong series is actually a Mario game. i understand why and how it happened. but this is a trivia that you either know or you would not believe from untrusted sources

It’s more the other way around: the first Mario game was a Donkey Kong game.

The user avatar is only “Mario” in appearance. He was called “Jump Man” in that game and even his outfit colours differed from the Mario games that followed.