> doesn’t even say that a byte is 8 bits

That is likely due to the fact the historically that wasnt the meaning of the word. There is a reason the RFC for IP uses octet to refer to 8 bit bytes. Another example of an old document that doesnt use byte to refer to 8 bit bytes is the Common Lisp standard.

So even though the current definition of byte is 8-bits the fact that the C standard uses the old definitiom is most likely because it is a document that can be traced back to an era were there was more diversity in the hardware architectures in common use.