I understand your question. I think the basic concept is that for user convenience, flatpaks are meant to be easily installed from a central repository.? "flathub" is the main one, and it is not expected that most users will want to archive a specific build.
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This in fact occasionally causes a few issues for KiCad - flathub basically has two channels: stable and beta.? There have been times when we wanted more channels; for example, when an RC of the 8.0 branch and an RC of the 9.0 branch are both current.? Which should the beta channel carry?? 8.0 or 9.0?
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But, that is how flathub is set up.? KiCad could set up its own hub and do something fancy, but then it would be a lot of extra work (and money) for the team to run a hub server, and it would be more work for users to potentially have more hubs configured.
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An alternative would be for KiCad to create stand-alone .flatpak files, and make those available for download on some server somewhere.? But most users would likely find it confusing to choose a specific file, download it, and install it, rather than just picking a channel and installing whatever is the latest version.? It would also mean more bug reports for the team if someone chose an older KiCad and reported bugs that had been fixed in a later version.
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So in summary, you can achieve what you want, based on the commands I showed above, but flatpak is optimized for the simple case of the official flathub server.
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BTW, I'm not the KiCad flatpak person - I handle the Fedora official KiCad builds, as well as the Fedora Copr KiCad builds.? So what I've written above is simply my understanding and opinion.? I hope it helps.
Steven A. Falco