This suggestion cannot be implemented under the constraints of different programming language's paradigm. Also, the only thing you can restrict when it comes to lifting up the challenge to the next level is probably code length, which fits well in a code-golf task, but not here, so closing.

But in O notation it will be 3x vs x² i think this is still better than having nested maps or foreach.

I think that all the language tools should be allowed

This kata definition should add an additional restriction for not using built-in language functions ? for example for php there is a array_diff function. Idk if this restriction is more oriented to the spirit of the kata.

Technicaly, the requirement didnt say EXPLICITLY that we could not use built-in function so there is a gray area. So despite this aproach is not "handmade from scracth" is also valid acording the kata definition.