I did not read that post about not feeding your chickens greens and my chickens certainly did not read it either. They'll eat almost anything green they can get at. The poster mentioning that may have been addressing a specific situation. I'd say let your eggs be your guide. If the egg shell is OK, your birds are absorbing enough calcium. And if they are thin, offer them extra oyster shell on the side before you stop feeding them some greens. The greens, especially the darker ones like spinach, kale, and chard, gives color to the yolk and extra vitamins and minerals to the eggs. Just like everything else, give them the extra treats in moderation, not as a major food source. Too much of anything can be bad.