Has it really been that long? Apparently so. Valve originally announced their rebranding of Steam Play with Proton back on August 21st, 2018. Seems like a good time for a quick reflection being halfway to a decade old now with the tech that gave rise to the Steam Deck.
Yes, they just started to pay who already worked on all that stuff and had the know how. They paid CodeWeavers to work on proton, started to pay the DXVK person, I’m not sure if the D9VK fork person was paid but I think so, paid the FNA person.
Though DXVK wouldn’t be possible without Vulkan and Valve was involved in Vulkan since the beginning. They also pay people to work on linux drivers since 2014.