Saying designers should code creates a sense that we should all be pushing commits to production environments. Or that design teams and development teams are somehow destined to merge into one team of superhuman, full-stack internet monsters.
The first time I presented design to a client I absolutely choked. I put the work in front of them and stood there like an idiot. It was humiliating. The next time was a little easier. And the time after that, well, you get the idea.