Why is fixed better on wet?

This last bit is true to a degree...if you had your rear brake applied it owuld most likely be worse than cornering on fixed.

i agree. if the rear wheel slips you are far more likely to catch it again as the wheel is still turning (if your legs are) but with a freewheel it just stops. also a bit of rear brake could lock the wheel but resisting even if it causes a skid will get traction quickly if your legs/wheel is still turning it's a bit like engine braking or the hill descent thingy in landrovers where touching the brake results in a slide but allowing the engine to turn the wheels slowly let it descend on slippery slopes.