You should let the game developers know also as they may need to configure their Steam cloud settings to allow this (or they may not want it to work).
I don't know about The Pre-Sequel, but with Borderlands 2 cross platform syncing is disabled because the windows client and the mac/linux client are not updated together, and this is a measure to prevent save file corruption.
We had this problem now as well. My brother switched from Windows to CachyOS and now everything is away.
More information: https://steamcommunity.com/app/261640/discussions/0/613939294499048925
Nothing extracted yet.
I started to play Borderlands: The Pre-Sequel on my linux pc, and Steam is syncing saves after I leave the game. But when I launched the game on mac, I realised that there are no saved data synced from the cloud.
Sorry for my english.