The same way that selling a timeslot that happens to be sized so that on average there will be one impression is not selling one impression. You aren't pretending to be able to accurately decide whether any given page load is an impression. You aren't even guaranteeing that there will be a page load, let alone an impression.

It seems like a way of directly eliminating fraud, by preventing people from having an avenue to lie about impressions, and a way of discencentivizing tracking, since impressions stop being the billing metric and therefore stops "needing" to be accurate. Hopefully this (as well as the other things suggested by OP to discentivize tracking) would result in less tracking and less hoarding of dangerous user data.

Agreed... Timeslots seem like slapping a technical restriction from a prior medium on the web for no particular reason. I'm not even sure what problem it's actually meant to solve?