The returned type is not a problem in itself, in this case JS coerces booleans to numbers, however returning only 0s and 1s makes the sort unstable for longer arrays (it works for random tests with arrays limited to 8 elements).

He probably dont even know, cause he just copied this solution:
https://www.codewars.com/kata/reviews/52aaf568fd48ce4e7b00068f/groups/58342174fa17ada46e0003e1
And made its 'look better' (shorter code = more popular here)

Your sort callback should be returning an integer and not a boolean.
For more details, check out: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort