This is currently flagged as Windows-only, but Twine games are almost always playable in browser – I don’t see why this one wouldn’t be. I think if you upload the zip file you already have and mark it as “playable in the browser”, it will Just Work because your entrypoint is already “index.html”. Give it a look at https://twinery.org/cookbook/starting/twine2/publishing_on_itchio.html#step-3-uploading-project-files