Also bear in mind the relative ease of porting between OS X and Linux (kind of like porting between iOS and Android) ...
Be very careful here. Most applications written for iOS are written in Objective-C, while most applications written for Android are written in Java. Yes, they share very similar underlying design philosophies, and some of the same underlying tech (OpenGL, posix), but porting can still be very difficult. Compiling Objective-C for Android would be a nightmare, and converting Java bytecode into something compiled for iOS is similarly hard, and that's after you write an API compatibility layer.
Mobile programs are only easy to port if they were written from the beginning with porting in mind, either by using an intermediate API and langage (like MonoTouch), or by writing everything in C and keeping the Objective-C/Java parts to a minimum. Unless this was planned, most people don't go this route, and porting those programs would be more appropriately called "rewriting".
The situation between OSX and Linux is similar (though not quite as bad). Porting from Linux to OSX is easy, because most Linux programs are written in languages also available on OSX. However, OSX to Linux is hard because, again, most OSX apps are written in Objective-C. You can compile Objective-C on Linux, and a lot of OSX APIs are re-implemented by the GNUStep project, but GNUStep is missing proper support for some language features that are heavily used in OSX. Which means that native OSX apps that were not written with porting in mind become extremely hard to port.
The good news in the specific case of Steam for Linux is that almost anything available on Steam for OSX is also on Windows, which means it has already been ported, and porting to Linux shouldn't be that hard. So yes, this statement is correct in this specific case, but your comparison isn't and "it's easy to port from OSX to Linux" is an extremely common misconception.