The only thing I could find on that is that spinach contains another substance that reacts with calcium and ties it up in a form that the body can't absorb, so extremely little of the calcium in spinach is actually absorbed by the body. On the other hand about 50% of the calcium in kale is absorbed by the human body during digestion, which is pretty high. I don't know if this is the basis for that belief or not.
Mine get a lot of kale during the season, which is mid spring to mid-summer. I have not seen any effect on egg shells. I can't get spinach to grow here well enough to make it worth planting.