Thanks for that link. The way I read that is pretty much how many of us suggest on here. Moderation good, excess bad.
Practically everything we eat as well as what we give to the chickens contains something potentially harmful. That includes things considered very healthy. The key is dosage. I'll use white potatoes as an example. Regular white potatoes (mashed, boiled, French fries) contain a substance, solanine that can harm people as well as chickens. But for you to eat enough solanine to harm yourself, a normal healthy human would need to eat about 50 pounds at one sitting. I can't eat that much. Your chickens can't eat enough to harm themselves either. But if those potatoes are sunburned and green, they have that poison concentrated to a point that they are dangerous. You can see a lot of warnings on this forum about feeding your chickens white potatoes. The dosage in regular white potatoes is not a problem, the dosage in sunburned green potatoes is. When someone warns you that white potatoes contain poison they are right, but they don't contain enough that it is a problem.
I would not be concerned about giving them some cress every day, just don't overdo it.