these were external developers. https://gigazine.net/gsc_news/en/20221219-steam-valve-support-proton-mesa
People are underestimating how much money valve has put into proton, Mesa and Linux development over the years to make the steam deck work. they had 100+ developers on payroll for years to do that.
top. you can see part of the handrail on the top left of the picture