I cleared up some of the mess. Hope it's better now.

It's a bad thing we can't check that coincidence

Nope, that’s absolutely not how it works.

The burden of proof lies entirely with the person who makes the allegations, i. e. you.

So it’s not “a bad thing we can’t check”. The only thing that may be unfortunate here is the fact that you’ve decided to jump to conclusions instead of substantiating your claims.

but the idea of having such problem is to actually code the solution.

So you’re saying the solution is so easy that it constitutes cheating?

Try Googling Rot13 Ruby you'll find exactly the same.

So you’re saying the solution is so difficult that > 100 people can’t possibly have come up with it independently without using Google?

If you think this is cheating, go code in C. This is Ruby, and it's done like this in Ruby.

Also, if the kata author have not actually disabled the relevant built-ins, it's considered not disabled no matter how the kata author says otherwise. As the old saying goes, show, not tell.