A wet runway is not the same as a contaminated runway. Airbus permits flex takeoffs from a runway which is simply wet.
A wet runway is only considered contaminated if the water depth exceeds 3mm. Most wet runways will not exceed this except during heavy rain conditions.
There should be a section in your FCOM that defines exactly what constitutes a dry, wet, or contaminated runway. As far as publicly available sources, page 82 of the Airbus Getting to Grips with Cold Weather Operations document defines them.
The document also clarifies the wet versus contaminated restrictions as they apply to flex takeoffs:
It’s possible some airlines choose not to allow flex off of wet runways, but this is their choice and is not an Airbus restriction.