Before you can say that a 66% success rate isn't good enough, you need to compare it to the human success rate. I barely get 2/3 myself.

This is my experience as well, very frustrating being locked out of your account when you need to take care of business. Nevermind cracked, they are fundamentally broken if a human can't get a nearly perfect success rate.

A captcha is cracked if it becomes economical to try to pass it over and over again. If you have a script that succeeds in spamming a forum 2/3 times it tries, you've got a successful spamming system.

What they mean by fundamentally cracked is that this method seems to be more robust against minor variations of spacing, font, etc. than CNN-based models.

Captchas are useless for their intended purpose if a bot can get it right better than every other time.