Princess Shirahoshi being the super weapon Poseidon is not that big of a plot twist/epic moment. It is not an epic moment for sure and I doubt its impact to the plot enough for it to be a twist. If you had to pick another moment from One Piece then Aoikiji leaving the Marines is a bigger plot twist IMHO.
Finding out Shirahoshi is a super weapon was mostly a shocker to most readers since most people thought it would be an inanimate weapon with a few exceptions. Its like when we found out Ichigo's dad Isshin from Bleach was a Captain class shinigami. Shocking but not a plot twist. Of course this is just my humble opinion.