> 80-120g protein as a minimum. That's a fair amount, and far more than you're likely to get from vegetarian sources at the time alone.

The article says legionnaires and gladiators were eating 5000-6000 calories daily, with 78% from primarily wheat and barley. An average ancient whole wheat would probably be somewhere near 15% protein by weight. Daily consumption of those grains even on the low end of their estimates would contain well over 100g of protein.