< and > in the context of [[ ]] sort lexicographically not numerically. By accident, the solution works nevertheless.

You can definitely make the argument that the average of an empty list is undefined because [0,] != [ ], and that returning NaN is more semantically correct. Saying it SHOULD be invalidated is a tad strong. There is a convention stating that the sum of zero elements is 0, but I don't know of any convention stating that the average should be 0. Imo an empty list can't have a well-defined mean.

Damn, I completely forgot about bitwise operators! Nice solution bro.

still clever for setting init to a double

Division of 0.0 by 0.0 is NaN. This solution should be invalidated and the empty array added to the test cases.

I feel like I always take the longest path with my codes when I see other people's.

This is really the smart and correct way to handle this kata.