That’s a bit different as the method used in that link involves storing the short URL. I’m suggesting that the short URL could be in an a-z+0-9 format, but calculated based on the ID. So there’d be some sort of algorithm which was able to convert from a standard base 10 numerical format into something utilizing the whole alphabet.

I’m not sure it’s worth the effort, but just thought I’d mention it since it’s the first thing I thought of when I saw the plugin.