Sure, but less grain is often worth it. There's a reason why fast lenses exist. The high quality lens being used here can probably still resolve 20 MP adequately even wide open.
I had that lens. It’s soft as fuck around the edges open.
Peak sharpness is about f/8. They should have had the D5 on aperture priority auto iso, pushed the exposure comp either way and then just fired at f/8 and let the camera make the decisions.
But they are astronauts not photographers :)
The modern Z lenses are far better and sharper open but much larger generally.