The main difference is conceptual.

Windsurfers are a minimalistic solution, with the crew providing most of the stability, rig and directional control. This concept is very complicated to scale up without giant human clones...

The proa's stability is provided by both geometry and crew weight, with separate rigging and control systems, so its concept may be scaled up with relative ease and also used in other multihulls.

I'd love to see a mega proa on foils for offshore racing.