But people are criticizing C because the way it is implemented is not "realistic". Or that the "hash" is a hack that will never work in real life.

I don't know the background of these people, but they couldn't be more wrong. So that looks like "morals" or just they don't ever saw real C code out there.

You're making an assumption that the C code is allowed to have the same algorithm as the Rust code. This is not actually exactly true, based on the rules of the game.

Where is it forbidden for programs written in C to use FNV?

For example https://github.com/haipome/fnv/blob/master/fnv.c

I'm referring to my reply to you above about not being able to implement a different map, not the hash function itself.