So not ROT-13.
A quick check shows that if you apply a (or +21 in the other direction), it may decode to something readable. tsryb fydyw nwdz ktkwtt msryt shrynyt... REPACK
It looks like the text you provided ("tsryb fydyw nwdz ktkwtt msryt shrynyt") is likely — possibly a simple shift cipher (like Caesar cipher) or a substitution cipher. So not ROT-13