If anything, it's the playstore and appstore which are side channels.
It's one of those seemingly innocent UI and communications changes that causes most users to develop a wrong mental model that obscures what's actually happening.
F-droid isn't actually installing the app. Neither does Play Store or Galaxy Store. Nor does Steam install your games on PC. People think they do, because the store fronts take over informing about installation progress. This little UI change alone - taking over the installer's progress bar - makes people develop bad mental models.
Direct installation is a great term IMHO. That's what you do when you download an APK onto your phone's file system, and then use e.g. file manager app to find that APK file, and run the system's package installer over it.
All F-Droid or Play Store or other stores do is to automate the "find the right APK" and "invoke installation" parts.
We could also imagine a mechanism to provide an update URL in the app metadata. The OS could query this URL periodically to check for updates.
So it's still a direct install, it's just that direct install support is limited on phones.