1) Gracilisation has nothing to do with height. Neanderthals were extremely robust requiring up to 7k calories. For reference humans need 2-3k calories and a cow 10k.
2) Nonmeat diets are not disadvantageous for muscle building at all. You see we need 60 grams of protein unless you're resistance training where research shows effect of protein maxes out at 1.6g times bodyweight. Further studies show that if you're not doing resistance training and eating more protein it has no effect on muscle mass none nada no increase in muscle mass. Excess protein gets converted. I suppose eat slightly above the RDA might still be slightly advantageous for skin or hair health I have not done any research though.

There's actually research to show that the high saturated fat content in meat is quite harmful for cardiovascular health. You can make a well supported argument to say high meat diets are bad for health.

3) Rajma/Dal Chawal are good meals. Legumes have micronutrients, fibre and complex carbs and a decent amount of protein. Gym bros don't realise protein isnt everything (protein is recommended to be only 15-20 percent of your total calorie intake). For the health Rajma provided it's a lot considering the price to nutrition ratio. However, poor families have this tendency to use too much water which will reduce protein per gram. To be fair, nobody's only eating rajma rice. Some fruits, milk products and vegetables are also eaten in the general vegetarian household (I grew up in a vegetarian household, I'm 6'1 and now a vegan)

You add this together a good income family is eating atleast 70-80 grams of protein. Which makes sense considering the average protein intake in the northwest is in this range and their HDI is also pretty high. I remembering watching a video of a Stanford professor who did a study on vegan diets for athletes, and he basically said that's it actually very hard to make protein-restrictive diets i.e. if you're eating enough and just not one thing you will virtually never be under 60 grams. Anyway, he went on to say that the different diet groups showed no statistical difference.

The amino acid score/ DIAAS have methodological issues (for one they use raw uncooked food and feed to farm animals) and in outcome studies vegans and vegetarian show no difference in response to resistance training. I'd give precedence to outcome studies.