There are a million ways to distribute an APK so I'm not sure where that parallel came from. I'm anti steam until they add decent support. I've dealt with marketplace issues too many times.
There is huge difference. Google makes it's harder and more user-hostile to install APKs from 3rd-party sources. They also create vendor lock-in ecosystem where half of Android features are only available when using Google Play Services that might not work unless you use Google Play.
On other hand Steam is just a software that works across multiple OS.
Marketplace is the one area where support is a negative; they can't be returning items to people because of accidental purchases, upsetting the seller which is likely just another regular gamer/steam user.
On other hand Steam is just a software that works across multiple OS.