For this to be true two critical conditions have to be met:
1. At any given time there is exact the same number of players playing each of the four factions,
2. Skill level of all these players is equal.
But that is impossible. I ma not sure that Is possible for any RTS game. Also, it would incredibly difficult to have the same skill level for all players ot be equal. We would need thousands and thousands of players online to even have a chance at everyone having exact skill.
I believe this is the closest we can get to thinking about balance in objective terms. Relic has not released any official numbers so this is what we have for now. Don't forget that this is data taken from the leaderboards