I think I've been quite clear in repeatedly saying that "learnt" was not wrong to use in the context that it was used in there. You've so far ignored that:
-
The Grammarly source that you linked yourself does not say that "learnt" is wrong
-
The dictionary.com page that the other person linked does not say that "learnt" is wrong
-
A word being the less common form doesn't mean it's wrong
-
A word being informal doesn't mean it's wrong
-
A word being irregular doesn't mean it's wrong