But not everything needs to look natural. Black text? Just fine. I do a lot of abstract art: Black works well. I'm not going to give up black ink linework any time soon.
I think this more subtle statement is fair advice to beginners in design, who might sometimes make an unreflected choice of black or not be aware that shadows aren't just black.
So in my opinion it's probably bad advice for more experienced artists like you, but it might be very helpful advice for somebody else.
The point is that this is false. In the vast majority of cases, you are not doing purely non-functional visual artwork, usability matters, and naturalness (or pretentions thereof) is undesireable.
It doesn't take long to explain where such rules came from and the effect they have. He's just parroting an "art rule" that isn't and then explaining some of his own process while acting like it is a hard-and-fast rule. I should mention that this "art rule" is the most commonly repeated "art rule" that folks repeat. I've had this conversation before.
(Seriously, I'm sitting here from the side of helping others people do art when they are a beginner, and have for some years. I've taught a weekend art class for children and have been making art since I was young - I started oil painting before I was 10. I'm over 40 now).
“Don’t use the flash on your camera” would be a similar rule of thumb for beginners, even though there’s lots of great flash photography and there are many circumstances where you’d have no picture at all without the flash.
I’d wager 95% of people probably go through their complete lives without ever thinking about how color works in visual media, and I appreciate posts like this one that may crack a few of those nuts.