They probably do, but all those are public companies. They all report sales a certain way. Google isn't going to mess with reporting sales that other companies are making. That's irresponsible.
No. Why would they? You think Asus, Lenovo and HP are giving sales data to Google?
They do know about Android activations, it's not so far fetched.
All the Chromebooks phone home regularly checking for updates, etc. Google knows exactly how many have been used and are in use now.