Strictly speaking, a flat plate is an airfoil. Like a symmetrical wing, it generates no lift at 0deg AoA. So you might say it is a sym. airfoil with near 0% thickness.

Really though, a lot of folks are flying these foamies to practice below stall 3D stunts, for most of which the airfoil shape matters next to none. For that, the flat plate wing works just as well as any, as the inefficiency (i.e. high drag) matters little at very low forward speeds.

It ain't my cup of tea, either, but more power to those who enjoys it. For that matter, I don't much more care for helis, either, but I'am not about to badmouth the heli-boys. They all bring $$$ into the RC hobbies, and that is always a good thing.