Actually he does because he plays the game at a high level.
Hugh level players know how the engine works as they use it to its maximum to exploit it to be good at the game.
Therefore they know which weapons, abilities, power ups and what not are weak and which are too powerful.
Shroud for example always goes against the meta because using the meta is too easy for him.
He uses a scout sometimes even a Mozambique in apex.
He will on purpose use crap weapons as a challenge because using the most powerful is too easy.
In fact I used to play destiny with 2 guys who would think about what weapons would be a challenge for them to use in trials using the meta was too easy for them.
High level players know the engine and weapon balancing better than the developers who made it. Because they are leagues above them in game sense and skill.
I'd like to see a developer bunny hop, destroy people with the worst weapons in the game, etc.
The best people to balance a game is professional players. In csgo the whole pro community wanted the krieg nerfed and now its been nerfed into oblivion. Its completely unusable now.
The problem is when a game developer only listens to its most popular streamers and you tubers and not every one of them for feedback.
That's where csgo got it right.
Games like destiny they listened to a handful of people then they decided that wasn't working so they would do it themselves and messed it up by completely nerfing the weapons that were skill based like snipers.
Now Dr D isn't a pro player however he is a high level player and will have a greater insight into game balance than basically all the people on here who have never played at a high level.
There brains are geared up with the game and engine. I play with a very high level player on Cs and I'm pretty high level myself at LEM but he is leagues ahead of me and has youth on his side. I watch how he clears a site in clutch situations and he knows exactly where the guy is without even seeing him. So after the round is over I ask how did you know he was there and he explained in great detail how he had checked x, y and z angle however at the same time his team mate had checked a and b therefore he could only have been 1 of 2 places and he didn't think he had enough time of the balls to be in the second place so he held point 1.
Me I would have checked some angles and then ran about trying to find him. It's that level of player who knows not only the map, what he has cleared but also what his team mate has cleared by checking radar and looking at there position on the map and screen and where the other person could be. They know exactly how much time the person has to get into position, exactly how much time it gets from a to b, its all natural to them they don't need to think about it they just know.
That is what seperates a high level player from the rest. They know that since you were here and 5 seconds have passed you can only be here or there and they clear those angles.
Whereas a noob like you would just run around and hope they run into them.
High level players are absolutely the best people to balance a game. However they can be biased so you need to talk to a large number of them and get all their feedback and assess it.
Knowing the current balance inside out, even the ability to breakdown how an engine and game mechanics work at a high level, is only part of the puzzle to understanding how best to balance and/or improve the balance of a game - maybe a mistake some developers make who pander too much to high profile players to the detriment of the game.
There are plenty of developers who can do things like advanced movement, etc. these days several game studios have active and/or past speed runners in the team, etc.