alt Hacker NewsToo lazy to dig up the PRs, but Flathub doesn't merge Windows applications using the Wine runtime unless the submitter is also the upstream maintainer. They don't mention this requirement anywhere on the docs, they'll only tell you after you've spent 12 hours getting it to work.
It sounds like in this case the submitter would indeed also be the upstream maintainer.
In any case, it's possible to install Flatpaks without Flathub (by distributing a single .flatpak file to be installed with 'flatpak install --user something.flatpak'), though this is obviously not as convenient. Would be interesting to see an alternative Flatpak repository specifically for Flatpak'd Windows apps.