Ooh I've seen that video before. I get the intent behind it, and I think it's pretty neat overall, but I agree it's generalized (though it would be hard for a short video to not be). No artist only makes works that are within just one level, unless they're really brand new and never drew anything since kindergarten.
I personally disagree about 4 being a "buyable" level, that would be at least a consistent 5 imo. Because of the amount of competition there is, getting people to buy your art requires more than just passable skill, unless maybe you're targeting a niche audience.
As for me, this video doesn't make me very optimistic about my art
I've been drawing "seriously" for several years, and yet I'm still only around level 4 - and that's on (rare) good days... I think my best pieces are around 5, like the ones below, but normally it's an unattainable level for me - it requires literally all of my effort. I've been like this for a few years already, and since I'm a self-taught artist with motivation issues and no work ethic, I'll probably never get much better.
My level 5 (?) examples