Nope, 2.5mm^2 would be fine, the maximum load could only be 26A max for a double socket outlet, and realistically 20A , so some if not most installation methods would have the capacity, exactly the same as a spur off a RFC.You can’t run a 2.5mm spur from a 32A 6mm shower.
Not matter what the Zs is. The cable is only rated at max 27A.
So unless you ran it in 6mm, which would be some effort for a 2g socket then it’s not going to be compliant.
Would not have to be 6, or 4mm^2, and if it was a single socket or FCU probably 1.5mm^2 would be ok from a load perspective.
I ask the question, because one's initial thought is to object, but is it actually wrong?
I know it's unusual and unexpected, but perhaps more compliant and less unexpected than some things we see!