You're right,...
I don't ride in rain much,.. and when I do it's by accident.
In my limited experience, motorcycles have very little grip in rain no matter what tires you use.
It's the two wheel thing.
A couple of square inches road contact front and rear then add water and you've got a recipe for extreme caution or disaster...
How much rain are going to redirect via a raingroove on a 2 inch contact patch of tire?...
Not much...
If I suspect rain,.. I take the car...