Rigged matchmaking has been the perfect scapegoat since MMR was introduced to competitive games. Every single popular competitive game has gone through this argument sooner or later, since it's just easier to cope with losses if you can blame them on something solid like a system designed to make some players lose, compared to something abstract such as luck, or even worse, thinking that you're not as good as you think you are.
Now, the reason why MMR exists and is used, is of course profit. If matchmaking is fully random, then Timmy who just started playing the game would get matched against xXslayer420Xx who's been playing for hundreds of hours, and poor Timmy would get so mad at being outskilled that he'd just never play again. This scenario is a complete loss in profits for the dev team or whoever owns them, because it's in their interest to keep every single player hooked to the game. Hence, MMR, where every player gets assigned a magic number that goes up when they win and goes down when they lose; and when they play, they get matched with other players in proximity of that number. It's not a perfect system, but it's easy to implement and it works most of the time.
The reason why I went into detail about the definition of MMR as I see it, is because it has an interesting little side-effect: once you reach your skill ceiling, the magic number stops going up, simply because you play against people of your skill level who might not have reached their ceiling yet. From this point, it can either go up in increments as you're getting better at the game, or it can plateau at a 50% winrate, hence the common myth. This is a fully natural process as a result of the implementation of MMR, and any company who would actually invest in enforcing a system that favors some players but not others would be straight up burning money.
Besides, talking about Battlegrounds specifically, I thought it's official that only the bottom three players have a chance at fighting the ghost?