I don't see why they'd need to make the marketplace fully open for their client to be open source. Fedora packages a Flatpak client that downloads proprietary software, but that doesn't make Fedora Sparkling Virtue Signalling.

Hell, Debian's repository now also include proprietary code (https://www.debian.org/vote/2022/vote_003) so binary BLOBs are perfectly capable of doing distro checks and refusing to run on forks.