Only looking at Polanco's 160 at bats in his specified sample vs Kepler's entire career is a terrible example.
I think you are the only one who read Thrylos' statement as "Polanco's recent numbers vs. Kepler's career numbers". Go back and read it again, that wasn't explicitly written nor do I think it was even implied (especially considering the same post invoked their similar ages, which implies the author is far more interested in their current place on the development curve than minor league career numbers, which very few people cite as meaningful, ever).
And frankly, Polanco compares pretty well, especially with the added potential to be a middle infielder. I have no problem with Kepler being ahead of Polanco, but I think it is fair to question how Kepler would be ranked #1 and Polanco #6 at this point.