I think if he was doing this for real (for profit) even than it would be questionable to target linux since its market penetration is pretty low while the effort to make you game running on linux is pretty big. I am saying this after spending some time trying to make a popular game running on linux. The lack of common API that _all_ of the linux distros support equally and easy to use for game developers does not help.
Valve stats on different platforms:
http://store.steampowered.com/hwsurvey
It shows an accumulated 0.89% market share for all the linux distros combined.