is that going to help on a mountain bike?
I would say yes.
A retul fit doesn’t tell you the exact geometric dimensions on how you should set a bike up. What it does do, based on my experience with my fit is that, with input from the rider it give you a 3d analysis of your riding style, from pedal stroke, body position, to lateral and vertical movement in your hips, knees, upper body etc. From that you can make changes to your position and set up based on what you want from your riding. i.e racing, sportives, tt’s etc. The idea is to create the most efficient way of riding.
Before even get onto the bike fit you are asked questions on past history and injuries. You also do a few exercises to establish how flexible you are.
This is based on my experience, but I’m sure someone can explain it more scientifically than me.