In context it's pretty obvious this is talking about omnibox suggestion rankings (which is also what changed and had the claimed "unwanted" impact), not search rankings.
Because I read more than random out of context tweets as my source of information.
> Is there another email that mentions Omnibox?
Yes. It's linked in multiple places in this very HN thread. Page 4 in the original justice.gov PDF specifically mentions the experiment that was done as a result of that originally quoted "complaint" email, and says "start experiments to improve search ranking in the omnibox".