It's "have you considered" rather than "you must do this".
The dev has the choice of if to commit the change that is suggested
That gets a bit lost in the game I know :)
To me the benefit of the default redundant arguments error is pedagogical. It forces me to think more closely about what it means to set nullable, assuming I was coming to the game with a limited understanding of Django code. Just glimpsing a note in the docs doesn't register nearly as quickly interactive play does.
I would love to see this format used more extensively. When I'm learning an API I tend to fire up a REPL and explore it by poking. I do it even with basic mathematics that have fallen out of my brain.
Very much enjoyed and it served as a quick refresher!