I would like to see a comparison between the top 10% vegan and omnivores. Most vegans are clueless when it comes to making up deficiencies in typical vegan diets.
Glycine is one of the relevant aminos that is very suboptimal or even scarce in vegetarian and vegan diets. It is also going to be one of the important (and likely most common) limiting factors in tissue repair.
Proline is also needed for collagen production, but not as deficient in Glycine as far as vegetarian diets. Glycine is also highly water soluble and a small molecule; it is constantly lost in urine. The need for glycine goes way beyond collagen production. It is essential for so many biological functions.
Glycine is not considered an "essential amino" because the body produces glycine itself, hence it is overlooked by many. It turns out the body produces only enough to usually barely scrape by. My guess is if another group of vegans were given Glycine (at around 4 to 5 grams/day) supplementation the disadvantage relative to omnivores would be erased.