He doesn't argue that porting the game is too difficult - in fact he implies that porting games to linux is pretty straightforward. He's really arguing that supporting the game after release is too expensive.

You have patches, and compatibility issues, and a thousand video cards with potential incompatibilities; customer service questions, system updates, and that one dude in Kansas running a custom version of linux that cant quite get your game to work, so he keeps calling and bugging you and taking up support time.

Edit: Found a relevant quote:

  This may sound ridiculous – “Who would turn away $20,000?”
  but the reality is that many of the same legal, financial, 
  executive, and support resources need to be brought to 
  bear on every single deal, regardless of size, and taking
  time away from something that is in the tens of millions of
  dollars range is often not justifiable.