The point made was that there will be a product key involved. They have to send you that, and they only do that if you preview 10 with a Microsoft account. That being the case, it's a simple matter for then to store which keys went with which account.
Clean installs are irrelevant. Being a local user is irrelevant. If you buy or are given anything online that needs activating with a unique key, it's its origin can always be traced back to you.
Uh, no. You don't have to provide any self-identifying information to create a Microsoft account and local accounts aren't accessible by Microsoft.
Sorry, but you fail. Try again.