Well, if it is solving CAPTCHA better and faster than humans, that is a useful signal. Maybe they monitor how often an IP address figures something out and if it gets too many right they +1 a suspicion counter. I dunno. Implementing the CAPTCHA solving bot also takes time and expertise; it'd filter out some low-ability spammers.
>The captchas are just solved problems being used to slow down bots and spammers these days.
If bot are solving these problems faster and more accurately than humans, it’s not really slowing down the bots and spammers anymore. That is also a solved problem.