That 10 million copies includes all the free ones they gave away on the Playstation. That's 10 million copies between Playstation, Xbox, Windows, Mac, and Linux. Some articles state about 40% of users were Playstation users with Steam users only being 30% [0]. Assuming the rates stayed the same and that an even number of Linux users bought the game as Windows (quite an assumption either way on both statistics) that would put the estimate of Linux Rocket League users closer to 20k, or 0.2% of the total purchased base. Overall, all these numbers are old and the final estimates are based on some big assumptions which may or may not be accurate.

[0] https://www.gamespot.com/articles/as-rocket-league-turns-one...