If you look closely, you'll notice that for each character, I square the difference. This is to to put extra emphasis on larger differences. If we don't do this, the string "Hannp" would have a fitness of 0. You see, the difference between 'e' and 'a' is -5, between 'l' and 'n' is +2 (which we have twice) and between 'o' and 'p' is +1. Adding these up yields a fitness of 0, but it's not the string we want at all.
Squaring is a dumb way to avoid that problem. You can still have false positives: for example if a string differs from the target by two letters, who distances are +1 and -1, then squaring will not mitigate the issue.
EDIT: I realize now that -1^2 is a positive number...
I'm very surprised that the author doesn't know about the Levenshtein distance metric http://en.wikipedia.org/wiki/Levenshtein_distance
That seems like it would be the perfect utility function for this case.