Obviously there is an additional cost; I think the point the parent was making is that the cost to add your third platform is lower than the cost to add your se

Obviously there is an additional cost; I think the point the parent was making is that the cost to add your third platform is lower than the cost to add your second platform, especially since one of the hardest bits (changing graphics stack) is mostly shared work between Mac and Linux. The cost is still non-zero, of course, and for the most part Linux gamers have to live with a lower degree of polish than Windows or even OSX gamers because of that.