Going by the Steam survey Linux is like 2-3% of Steam's userbase. Not even allowing for how many of those people do have a Windows PC on the side, it's gonna be a while before you convince devs to stop having Anticheat for the sake of that market segment specifically. If they were going to balk at the losses they would have added support in the first place. Being maximalist about it is fine as a principled stance, but you don't have enough of a wallet to vote with it on this one.
The comment said "backup OS" because the video ends on a complaint that GRUB got pretty badly broken, presumably by the way they set up their Windows dualboot. I was clarifying for the record that he also suggested dualbooting for other reasons, mainly software compatibility, before things get to that point.
Again, he's not wrong on either count.