It's incorrect because of consistency issues. Sum works with every overloaded + operator. This is not a Haskell thing nor is it a "personal definition" of correct. People expect a definition to be consistent, this is a generally prevalent notion that transcends the boundaries of "personal" as the general population agrees with this notion.

Due to this, under a general definition of "correct" without the context of Haskell such a thing is still classified as generally incorrect. Instead it is more accurate to phrase it as "this is Guido's personal definition of correct." Not that there's anything wrong with that but this is generally unusual and opinionated behavior encoded into python and I wish people will acknowledge that.