First: “gamers should be free to engage in any game with their friends anyplace they want without any unnecessary friction.” This is a bit ironic coming from the guy who shut down Rocket League on macOS/Linux. I don't have a Windows machine and I don't want one.
Second: Why should the makers of a mobile OS allow a method for sideloading? And why in the name of all that's holy should I, as a consumer, advocate for this? Whatever you think about the security of pre-vetting apps in the OS default store, there is at least SOME attempt to ensure the application isn't a complete pile of virus-laden garbage. My mobile device is the computer I use the most often, the one that tracks every single place I go, that is used to manage 90% of my communication, it's my payment method, it's where I handle most of my banking actions. It's my camera, it's my GPS, and it's my calendar. My home gaming console is a device dedicated to playing games. One of those devices will totally hose me if it is compromised, whereas the other will cause me some minor annoyance and maybe regret over a lost saved game. These devices are not equivalent. These devices absolutely should NOT both use the same security model. It is ridiculous that anyone would claim a locked-down, secure OS is the wrong approach for a mobile device because of inability to play a free-to-play game like Fortnite.
He simply does not want the platform to get a cut of the sales. End of story. I wish he would at least have the integrity to say it, rather than couching it as a noble attempt to push for user freedom.