Nutrient density and bioavailability are two very different things. Kale is extremely nutrient dense but as the parent poster has pointed out, the anti-nutrients [1] reduce the absorption of said nutrients.

You can take kale into a lab and determine that it has so much of nutrient X and Y but in reality, do you absorb it?

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4663599/