5 makes perfect sense to me; the author's complaints seem kinda silly.

An area this makes sense is, what do you expect to get if you do something like:

    emoji = " "
    print(emoji[:3])

Should this throw an error because there's only one displayed "character"? Should it return only a partial codepoint by returning only the byte data for the first 3 bytes?

Modern strings are complex objects that have evolved a bit past char[] or byte[].