Problem is its easy to cheat about whats yours
Even at the ML level, engineering is more about applying known good techniques and less about new innovative ideas.
You can't fake this. Or to put it better, even if the code isn't yours and you can do this well, it doesn't even matter that the code isn't yours as in doing this you by definition have the skills and knowledge to reimplement it anyway.
Have you actually tested this out?
Regarding your other point I agree with you, but if the main point is to evaluate understanding then the difference doesn't really matter. If you're testing for ability to invent concepts etc, then this might be more important to you.