Byakugou no Jutsu is just a permanent version of Creation Rebirth. Why would Tsunade use the latter if she had the former according to you? As far as her pt.1 feats are considered she didn't have Byakugou no Jutsu to that time.
Tsunade would use Byakugou No Jutsu instead of Creation Rebirth if she truly had it during pt.1
Maybe she developed it right after the battle against Orochimaru.