Fine. As well they should.
They advertiser can work this out with the website the same way I would have worked it out with the Miami Herald in 1955. Frankly, I'm not sure if that was trusting the newspaper's ad sales department to be not fraudulent when reporting circulation numbers, or if there was an industry group like Nielsen that verified such things or made estimates based on 3rd party polling.
Whatever the method, plenty of ad sales were made, and everyone was happy, and I know it didn't involve 100% tracking of individual readers. Alexa can either adjust the model that they have to account for the growing number of people who block their web bugs, come up with a new process or model entirely, or die in a fire. I don't particularly care. Life went on before all of this big brother tracking shit, and it will go on after it is defeated, or at least marginalized.