While I do think there are some wild alegations stated which would be impossible to prove, there is some anecdotal evidence, I think.
I have noticed and wondered about the platform population. It does seem odd. There are so many times that I have started a match of breakthrough with crossplay on, and noticed that everyone playing is on an xbox like me. I have thought, what a weird coincidence and thought maybe crossplay is broken today. And then later, there are PC players in the match.
It isn't proof of anything, but it is weird that the match adds PC players in later, not at the beginning. And it's not a one-time observation. And now I see that others have seen the same thing? It is strange.