I don't have the DMG, but is it true that treasure is assigned by CR?
How does this affect creatures like dragons who typically have hordes of treasure, not because of their CR, but because hording is part of their nature?
It's a pretty loose system. It even states you can roll more for monsters that horde treasure. Much more of a guideline than a straight CR = this treasure.
I liked the guideline for providing common/uncommon/rare/very rare/legendary items to players based on level as well as they're caveat that hey, do what you want if it works for you. The treasure charts seem to parallel this mechanic.