The Irish agricultural development board knows what it's talking about. Here's the pdf which you apparently didn't bother to read (and yes it's about cows, but I've yet to find anything from the pastured poultry sector that compares). No-one is suggesting that chickens can live entirely on...
https://www.teagasc.ie/media/website/animals/dairy/Whats_in_Grass_Todays_Farm_May2014.pdf
edited to add, for those who don't do pdfs, the nonsense is revealed by the bit that says "The energy demands of the dairy cow can be met by a grass only diet throughout the main grazing season, with some...
Your powers of deduction are noted.
For others, this might be of interest
https://www.backyardchickens.com/articles/grass-for-chickens-taking-a-look-at-the-natural-health-benefit-provided-by-this-everyday-green.67397/