There is no way to know what that function requires as input.
Of course you can't know. That function has no requirements. As such there's only one thing it can do.... return x. Anything else is making an assumption about the parameter being passed.
Great article I think.
I don't have a lot of experience in zig, but I feel like it's just a better version of C. More specifically, C with a more modern synthax, better defined behaviour, better error handling.
As the author highlights it, using the comptime and reflection to make generics can easily become a footgun and make the code messy. But hey, having the option to make generic code is still better than C.