Yes it's indirect, but remember that Microsoft is one of their biggest competitors. This isn't about seeing absolute profit from every change, it's about improving linux as a platform to make it more viable for consumers, which will make it more viable for developers, which pushes more people to Steam and SteamOS as their first Linux distro and first destination for games. By making the platform perform extremely well on older/cheaper hardware they also create a market for other businesses to create hardware (Legion Go for example) which will increase the PC market and increase the number of people using steam since it's the defacto monopoly. Yes, they won't necessarily get every penny from every sale of hardware or even games since other game stores exist, but they will get a huge percentage from the majority of people in the PC market.