Why is fixed better on wet?

Thinking about it for me, with brakes you have to scrub speed before you turn in the wet. If you misjudge the speed and are too hot you then have to scrub more while leaned in you run the risk of the rear wheel breaking away. You're far more in control of your speed throughout the turn and because of that run less risk of disrupting the grip of the tyre through quicking slowing it. Not scientific, just a thought I had the other day blasting through a turn

If you run in too hot your more likely to loose the front than rear. The way to slow in the corner is a smidgen of rear brake, or (when riding fixed) slow your legs a little.

They same result is possible.

Your assertion that there's less risk of disrupting the grip of the (rear) tyre through quickly slowing, assumes you don't skid the rear accidentally when slowing your legs, and therefore that it's a more subtle braking method than a lever brake.

It's a question of how good a rider you are on the brakes... and how good you are fixed.