Well, but there are a lot of CPU mining coins in crypto space. Anf how do you be sure that only one prson will be able mine here? For example I can have 3 copmuters/laptops and mine with 3 PCs etc. 
You wont be able to verify 3 accounts at the same time. Maybe 2 but only you will be lucky and fast enough.It is not only captcha solving but also making, within limited time, not to mention you will need couple invite codes.
In short, this system appears to be working quite well, no cheaters were spoted so far. Besides it's not actually CPU mining, it's proof of person, you get paid by being a human, not by having the fastest equipment. Give it a shot and see for yourself.
What do you mean? Why in the same time? Why dont I can mine from different PCs for example? Whats getting invite code? Are this codes giving some % of rewards?