I would add that this actually significantly improves the user experience in scenarios where there is mixed usage between the the GUI package managers and the dnf command line.
Currently when you are installing packages or updates in a GUI package manager, it isn’t possible to roll back or even list those in dnf, because they are only in dnf4’s transaction database, which trips up users all the time (just check the community Discord and/or Matrix rooms).
For the same reason this also solves the problem of <unknown> package sources (From repository line in dnf5) and therefore also directly handling of the autoremove command. Because for packages installed through the dnf4 backend, dnf5 doesn’t know how or for what reason a given package/dependency was installed.
Also took me a minute to find where this backend is coming from, since my most recent info was that there isn’t one, but I guess this explains it