I think WASM doesn't really allow OpenGL calls directly, at least not through the C API, but through WebGL calls.
I don't really know how you make JS calls from WASM and vice versa, but obviously that would be painful to adapt SFML to WASM since you would add js code to make WASM work.
I don't think WASM will allow direct C OpenGL calls in the future (but I really wish it could happen), seen how complex it is to map C functions to system/driver calls. Peeking at what glad/glew does is enough to understand that things are not simple, seen the multiple opengl versions, core or compatibility, chip bugs, etc.
So in short, my former question was naive and uninformed, but there might be details I'm not aware about.