Bot detection systems all work client side. What browser are you using, what cookies do you have, how is your mouse moving, how much time are you taking between clicks, how many captchas have you solved. All this and more is collected and passed to the server, which can then determine if the view was valid or not. Plus you can do this multiple times during a watch session rather than just in the beginning when the video is requested. If the adblocker is blocking this data then the server has nothing to go on.