Nobody here is arguing from direct information, just implications of vague statements. Here's where they spell it out in more detail:

https://forum.unity.com/threads/unity-plan-pricing-and-packaging-updates.1482750/

Q: How are you going to collect installs? A: We leverage our own proprietary data model. We believe it gives an accurate determination of the number of times the runtime is distributed for a given project.

Q: If a user reinstalls/redownloads a game, will that count as multiple installs? A: We are not going to charge a fee for reinstalls. The spirit of this program is and has always been to charge for the first install and we have no desire to charge for the same person doing ongoing installs. (Updated, Sep 14)

Note the update there. They completely walked back their previous answer:

Q: If a user reinstalls/redownloads a game / changes their hardware, will that count as multiple installs? A: Yes. The creator will need to pay for all future installs. The reason is that Unity doesn’t receive end-player information, just aggregate data.

Which has lead to a lot of confusion. It seems like their "proprietary data model" is focused on another point, which is preventing install spamming. Or maybe it's also about reinstalls, even though they "don't receive end-player information" so that was impossible a few days ago.