I would wager that the reason Marco (and many other mobile entrepreneurs) gravitate to iOS boils down to one line in his post that he doesn't feel the need to defend:
"We can make a living developing for it."
Android has (roughly) 4X market share but just 1/3 of the revenue compared to iOS. When the per user revenue gap closes, Marco's preference may change.
He already has a version of Instapaper for Android, so he clearly has no problem making money off the platform, whatever little it may be in comparison to iOS.
He's also gone into great detail on his blog about how unsuccessful that has been and if he worked on the port himself (outside company volunteered to work on it) he wouldn't have been able to go fulltime like iOS allowed.
"We can make a living developing for it."
Android has (roughly) 4X market share but just 1/3 of the revenue compared to iOS. When the per user revenue gap closes, Marco's preference may change.