'Mixing up red & green' is one type (or a trait of some types), not the only one, and it doesn't mean say 'RGB' spans only two colours - it means some shades of red appear green and vice versa. It can also depend on context (red laser pointer invisible over a sea of green, say) and knowledge of the difference (presenter says something about their red laser pointer).
There's no problem with using green, it's requiring it to contrast with red that's potentially problematic. For that reason it's probably a good general theme/background colour, even if that was the sole reason they were choosing one, because you'd be unlikely to want a contrasting red text colour over green background anyway.
it's not about choosing only one colour, colour blindness and colour systems are more complicated than that.
If you see the image under the "Double the tests, double the success" subtitle you'll see that the blue colours were actually failing a few of the APCA tests for colour contrast.
So why would a green design be better than a blue design? Naively, it's strictly worse.