Must APPLICATION_IDs (package names) be reverse DNS?

I always thought that com.google.android.packageinstaller or pm would refuse to install an .APK if its manifest didn't include reverse DNS for the package name. However, adb shell cmd package list packages returns package:android within the list of standard package names. It's the sole exception, which actually makes it more startling. Within comparable package managers which utilize such a structure (like flatpak) the rDNS is mandatory in the package manifest. I expected pm to be the same. Consequently: - Is this rule true for applications which the user installs but not the system, perhaps? Or - Can a package merely be named anything, yet the convention exists due to the Play Store, or another driving force within the ecosystem?