@pixelherodev This argument sounds backwards.
I think github is detrimental to open source and wants to lock you in, and like the other comment says, I think they do offer you a proper diff if you know the undocumented URL, but as for the argument you're making:
If they make you use git to get a proper diff, isn't that encouraging you rather than discouraging you from using git? It's when they offer functionality in the Web UI and discourage you from using git itself (like git's built-in PR functionality) that they're enhancing their lock-in.