This is simply not true in the real world, as discussed in the last paragraph. Secondly, it assumes that there is never a half-hour gap in a genuine visit. This is also untrue. I quite often follow a link out of a site, then step back in my browser and continue with the first site from where I left off. Cookies don't solve these problems .
Some sites try to count their visitors by using cookies.
But this can only work if you refuse to let people read your pages who can't or won't take a cookie. And you still have to assume that your visitors will use the same cookie for their next request. You can't follow a person's path through your site . It's very common for people to go back to pages they've downloaded before. You never know about these subsequent visits to that page, because their browser has cached them.