John, I agree that he can use platform detection, but he should still list all three there. How many times have you gone to a site’s “downloads” page and gotten every flavor listed? Since, technically, you could be browsing his site on a Windows box but want the app for your Mac box (uncommon scenario, but still very real and possible). I would personally use the platform detection to make the user’s platform the one showing at that moment, but still leave the other two options there.
Alternatively since he doesn’t seem to be offering it to Linux other than HTML5, he could always default to the HTML5 one.
Haven’t had my morning coffee, so my thoughts are a little scattered. There’s two viewpoints.