The purpose of 'eating' before a workout is to reduce the body's catabolic (cortisol) response to stimulus. BCAAs, glucose and types of fat can reduce the response by several orders of magnitude, leading to better-feeling workouts and increased rebuilding response to strenuous exercise.
There is nothing magical about BCAAs:they have a low calorific density, meaning they don't alter body chemistry too much before going for a workout, despite providing the 'right' nutrients for the body's impending stress.
Measuring impact on DOMS is hard, because the body adapts to the exercise regime over the period of 1-3 weeks (depending), so DOMS naturally reduces in severity over that time. In other words:
"Try some deficit Bulgarian split squats - your legs will thank you."