Untrue. You can install whatever you want on Samsung's android variant.
This change makes it so you can't install software (such as F-Droid, NewPipe, Google Camera, Samsung Notes, etc.) from APK, unless you install them directly from Google's Play Store [without going through unnecessary hoops and 24-hour delays].
And, pretending "Samsung has been doing this for ages", it doesn't make it right.
[Edit for clarity]