Discover is an app store, not a full-featured package manager, it isn’t designed to show all individual packages, nor most command-line tools. I also believe it depends on a package having correct metadata included, and this particular package may be missing this on your distro.
This is the normal Ubuntu 22.04 package, which looks to be the latest git pull at one point, back in 2018.
If your distro’s package is really that old, then it seems likely to be missing the metadata that packagekit/discover/gnome software needs to show this application. Some software applications don’t show in *buntu for similar reasons.
I don’t know why the updater shows all (it probably is just displaying a list that the system’s native package manager shows as upgrade-able) , but it would be silly not to list all the updates, even if they are just system libs and such.
But the search and display portion only shows apps that have more than just a basic description, via appstream metadata. Discover and Gnome Software are not intended or designed to be full-featured package managers, like Synaptic, Pamac, or Yast.
Think you could elaborate at Why does Discover only show applications with AppStream metadata?? It’s really difficult to believe that that could be so, considering that even the Microsoft Store and Google Play Store allow their users to view dependency packages like the MSIX VS C++ runtimes, or Android System WebView packages, respectively.