Considering it's two high profile players, I'd say the most likely is that they were tricked into downloading something, or some other software they were using had an exploit (I've had one from a browser plugin before now). There's a video elsewhere in this thread of one of them downloading Malwarebytes for something, so maybe they didn't manage to get rid of whatever it was.
Other option is an exploit on the server. Maybe there's some way of sending malformed data to a player you're not currently in a game with to exploit an RCE. It's not completely impossible, but I figure we'd see it a lot more if that was the case.
I'd put money on option 1 though.