>Or worse, perhaps they did get this from a shelf of existing solutions.

I'd like to believe that was impossible but of course given how irrational it would be to go through the trouble of making this, it's probably likely. Though I don't know why you'd necessarily skip over recaptcha and securimage and how far down the list you'd have to go to get to this sort of thing... but ZeljkoS has a couple more examples like it on his site. So apparently it's a captcha anti-pattern.