In the grim darkness of the 401st century, the Imperium of Man has an unwritten rule about the small area on Earth that is still green and forested, and about the strange old man who lives there: They do not talk about that area, nor about the Merry Fellow who lives there!
They especially do not speak of the time that the emperor went there in person to try to impose his rule on that land, only get insulted with silly rhymes, and then have his golden-armoured butt kicked flying out of Bombadil's self-declared realm.
No, they do not talk about that at all! It Officially Never Happened, okay?
The really grimdark thing is that old Tom could easily snap his fingers and erase all of Chaos, make the Tyranids crumble into space-dust, put all the Necrons back to sleep, etc etc... if only he could be persuaded to give a moment of concern about anything that happens outside of his little pocket kingdom.
But don't try bother Tom about things like that. Tom has his house to mind, and Goldberry is waiting.