Valve launched Steam Machines with their own OS and started shipping a version of Steam on Linux with predictable library versions. At the same time, they started working with the Wine project and shipping things which is now called Proton but is actually the cumulative results of their own patches.
This paved the way for the success of the Steam Deck when adequate material became available.
I don’t think it makes sense to call the Steam Machine a misstep because there was no Proton. There would be no Proton nor Steam Deck without the ground work started with the Steam Machines.