Is it processor architecture? Is it the OS? Is it the store and whatever facilities they provide? Is it the mode of physical interaction with the device (desk, couch+TV, etc)? Is it being able to assemble any random collection of hardware and expecting it to work? Does that discount set builds? Is it mandatory that the user is free to screw around with the software any way they please or can you lock stuff down?
At least on the commerce side, it's been the case since steam was opened up to third parties that they were the gatekeeper for success unless you were already huge (and now very few companies want to go it alone). Going back to Introversion's Darwinia they were just scraping by until they got on steam, developers have long been complaining that the varied methods Valve has used to get on the store (manual review, greenlight, etc) showed the vast majority of gamers only purchase through it or that you'll get a large wave of new business when you release on it. Now it seems like a 'tragedy of the commons' situation unless you've got your own marketing or it's a hobby project.
It seems like you've now got to do mental gymnastics to say Valve doesn't own the PC gaming platform