Oops; you are right. If my memory serves right, showing prime factorisation in P will imply P == NP; which would have been a sensational news at a different level altogether.
> showing prime factorisation in P will imply P == NP
Whether "Factorization is in P" actually implies "P=NP" is another open problem (most researchers in this area don't believe that this is the case).
What does hold is that if we found a "fast" algorithm for factorization, this would break some cryptosystems. The most well-known example is RSA, but other, more academic cryptosystems would be broken, too.
Well, it is also not proved to not be NP-hard, so it could also be NP-hard to our current knowledge. But you are right that researchers believe that it's not NP-hard.