I think you would be wrong about that. Certainly, it's not just WASM that makes Figma fast, but it's an important piece. Look at it this way: why would they have built out the product with WASM five years ago if Javascript was just as good for their use case? It's a huge investment in a new technology with relatively few people you can hire for it, versus quite possibly the most well-known, well-supported language in the world. They identified some significant advantage that made the cost-benefit analysis line up.

Normally, I don't like the argument that because somebody did it, it must be the right choice. But, Figma made so many correct decisions out of the gate that, in this case, I'd give them the benefit of the doubt.