Nema 14-50 40A outlet for electric car: one gang box (plastic or metal) and cable size ...code?
I'm betting this is a Tesla charger, which draws 40A. You cannot load a 40A breaker at 40A continuously (defined as 3 hours or more), you can only load it to 32A (80%) because technically, a 40A breaker is designed to protect 40A rated conductors, which by code must be sized at 125% of the continuous load. So you need conductors rated for 50A and a 50A breaker, it wasn't a suggestion on their part. If you ran #6 conductors and wanted to protect them with a 40A breaker, technically that would not violate Code, but you are likely to have nuisance tripping of that breaker. That's a nasty surprise to find out about in the morning when you want to drive to work...