Tim Fortnite is arguing that games sold on Steam should be able to offer in-game purchases with payment options outside of Steam.

But they already can and already do. For example If I wanted to buy ARX for Elite Dangerous, you have to go through Frontier’s website to purchase it. Same for Daybreak cash for Planetside 2. And isn't Maplestoy also on Steam? You most certainly have to kiss the Nexxon ring before purchasing NX.