I would say Davis's definition of "objectively better" here is "nobody who reads these poems carefully could possibly conclude that this AI crap is better than Walt Whitman, the only explanation is Walt Whitman is so difficult that the raters didn't read it carefully."

The Nature paper is making a bold and anti-humanist claim right in the headline, laundering bullshit with bad data, without considering how poorly-defined the problem is. This data really is awful because the subjects aren't interested in reading difficult poetry. It is entirely appropriate for Davis, as someone who actually is interested in good poetry, to make a qualitative stand as to what is or isn't good poetry and try to define the problem accordingly.