I’m assuming you copied the hint; the hint is wrong in Chrome as of October last year; it’s been updated but the new version of the hint isn’t live on the site yet.
The callback function you’ve specified is not correct, that’s why it doesn’t work – it needs to return a negative number, zero, or a positive number, not just true or false.
As @LiChoi says, just using the default with no callback function works (and this is actually what the hint has been changed to). Note it does not sort alphabetically, it sorts by string code values, which is something different (here are the first 255 code point values for example: https://www.rapidtables.com/code/text/ascii-table.html, that’s what it sorts by).
In this case the sort order is alphabetical because all the characters are lowercase letters in the English alphabet (try adding some uppercase letters, & you’ll see it won’t be alphabetical – it’ll order all the uppercase letters alphabetically first followed by all the lowercase letters alphabetically).