Oh I don't disagree that a good DMG will have useful info for a new DM. I am, however, skeptical, of how much it can actually "teach."
What does that even mean though?
If people actually read it, a book can give them some really great resources on how to DM.
Part of the problem is that the DM's Guide for D&D serves about 14 different masters. You could give people the information they need to have a good basis to be a good DM in like, 16 pages or 32 maybe, and it's largely system-independent stuff. But the way WotC organise things, that'll likely be either split among a dozen different chapters, or put 3/4s of the way through the DMG, when it should be absolutely the first stuff you read as a DM.
EDIT - to add to this, there are really only three sources I learned from positively when DMing in my first few years.
1) The older cousin who ran AD&D for us and left an adventure for us to look at how she wrote it. She also gave me some basic advice which was way ahead of its time, frankly.
2) RPG books on DMing.
3) The early internet.
I don't think the "folk tradition" thing is super-helpful because so many DMs are so bad. If I'd learned from the first DM I played D&D with after my cousin, who was almost the first DM I played with (only the summer holidays prevented that being the case), I could have ended up as a totally awful DM because he was absolutely terrible and tyrannical and eventually got deposed by the players. One of those real GMPC-loving, player deprotagonizing scoundrels.
It's true that now you can see DMs running stuff in Critical Role and the like so that makes the "local weirdo" factor less bad, but a lot of the popular DMs to watch are so far beyond anything a new DM is likely to be able to do skill-wise that it's kind of nuts.
I mean, maybe the best argument for "not a book" is that "kids today" often prefer to learn from videos than books or written information (apparently they're worse at Googling than us Millennials too), but that just means WotC should think about making a video series to accompany the DMG.