I just checked something... did you try the playable ad export? Because that only generates a single html file which includes the box2d wasm (as base64) so there wouldn't be a wasm file.
Yes, I've tried and in this case the error is that the file is big to upload. (it's about 3 MB, but per their requirements they allow up to 5 MB for all files for a zip folder). So again it's confusing)