TL;DR: because mid-20th-century designers believed soft green reduced eye strain and improved focus.
Basically the same nonsensical belief as in regard the dark mode nowadays.
I don't even believe it's true. Green is just an army colour, that's pretty much it. Army uses army colours. Mystery solved.
It's the color of plants. A field of grass. Etc.
Maybe it even works better with the color of a clear blue sky above it.
Anyway, it's intuitive and not rocket science.
As the son of a machine tools salesman, I call the article bullshit. Sometimes things just need to be painted and sometimes you just need that WW2 surplus paint to do the job, with the colour not mattering one bit.
With anything, an academic can thread together a theory that neatly joins the dots to sound feasible, but my bet is that 99% of all engineers are stronger at physics than color theory.