I think Google's motivations are likely to include protecting the internet, as they are the incumbent in so many spaces. Problems honestdns solves:

First, outright faking of dns resolution. Maybe to switch ads on internet pages. Maybe to falsify the pages entirely. Remember how internet in hotels worked 10 years ago ?

Second, lying to improve cost metrics for isps. Say, lying to cdns'es about the users location to use the cheapest connection for the isp. Or just lying to give users a bad connection and save on bandwidth.

Third censorship. Mostly dumb organisations' censorship. School networks in early 2000s are a good example.

Fourth, special support for a number of their products. Starting with, of course, their own cdns, but I don't think it'S limited to that.