It's possible to nurse FD along in the wet, particularly on flat ground without grades or hills. More attention to spindle down pressure, easy feathering of the throttle, no sudden accelerations and ease up to speed. It takes a different touch than when the surfaces are dry.
That said, all FDs that utilize a steel spindle will slip on a wet rubber tire for the same reason bikes with steel rims will have poor braking with hand brakes: wet rubber slips on steel.
I've logged about 3000 miles this year on MBs. I have the BMP FD, several actually, as well as a Staton and DE also. All of them with a steel spindle can and will slip in wet weather. Granted, where we live there is a lot more hills than FL but steel spindles will slip even when it's not raining just in wet leaves this time of year. About the only FD I know that won't slip in the wet is the DE with the stone aggregate rollers but they wear tires quickly once the road surfaces dry out. It's just one of the trade-offs with friction drive.