10 million is just an arbitrary number he will not honor when it is reached.
Valve has sold ‘multiple millions’(source) already. The 10 million will probably be reached soon. Not even to mention all the Linux users.
And frankly it’s not a him problem. Nearly every dev refuses to release on Linux (and Mac) because of its small user base.
Yes it is. He does not have to release for Linux. He just needs to allow the anti cheat to run on Proton. This is a simple config change not more. Fortnite will probably run fine on Proton.