Okay, here goes:
1) I see green in those videos (other than the cabbage)
2) It appears they are out free ranging?
3) They have never seen a whole cabbage before
4) They are CHICKENS
(i.e. new things are deadly!)
So, why would we



want to nibble on something potentially dangerous, that we have no idea what it is, when there are plenty of greens about, plus I can scratch up a few bugs (YUM) too.
Suggestion: Hang it in their run/pen when it is too miserable to be out. Cut it in half, so there are 'edges' that are easy to peck at/off. When hanging it, use your treat voice/call. And if all of that fails, take a bit of peanut butter, smear thinly on the outside, and sprinkle with scratch grain or some other small treat that they like and will stick. Finally: give them a day or two to be comfortable with the scarey new thing.
Don't thank me....

you'll be cursing me when they devour multiple of them in 5 minutes, and you have to spend a fortune to "keep them in cabbbages!"