If you already own the game on Steam just use Proton. It ran perfectly fine for me through various proton versions over the years (whatever was the latest at the times I played) or proton expiremental. Before they dropped native support I would typically run the game through Proton anyways as it performed much better than the native Linux version at the time.
Otherwise using Heroic game launcher should be fine. Prior to Heroic getting popular I saw that people were installing Epic Games Store via Lutris and that ran Rocket League fine too.
Yes, SteamOS does count as Linux. Android does not. The Android and iOS Steam app is just for social features / store, not for playing games so neither show up on the survey.
SteamOS Holo, which is what the Steam Deck uses, makes up 42% of the Linux systems in the survey results.