non existent domains are the ones that are most likely to be somehow personal to the user, because they weren't trying to enter a domain at all but it got interpreted as one accidentally. Eg a password they meant to type into a password field but the url bar was highlighted. If they were interested in statistics regarding popular domains, like google or facebook, then it would actually be less of a privacy intrusion, because it would only end up telling you about populations, not individual users.

I don't know what they actually intended to use this data for, but its telling that they don't mention that in their proposal.