I caved in years ago and registered. Only to find out that the site was still horribly broken and it was impossible to get any information out of it. Truly a 'Fuck You' pattern.
It’s fairly trivial to detect bad data, especially when you know what good data looks like. And once you’re in, they can still get useful data even if you lied.
For one, how carefully did the user input information into fields. If you're making it up, it will be much faster and less hesitant than a legitimate entry.
That and a hundred other data points help create a ML model that reliability identifies illegitimate activity.