You aren't missing anything, but do understand that rules are meant to be a guideline and the ultimate Rule 0 is that the DM has the final word.
It should go without saying that a DM can always override the rules.
But there's a big difference between "this is how I run it in my campaign" and "this is how the rules work".
The post that started it all:
Um no, this refers generally to stuff like armor and helmets and boots. And items only resize within a single size group, such as Medium for PCs. So a suit of armor will magically resize from a small halfling up to a big half-orc, but not down to a pixie or up to a giant.
Pretty much all of that is pure house ruling, which is fine. But there's 0 mention in the DMG of magic items only resizing to fit humanoids, size restrictions or anything in that post. In fact they go out of the way to show how items could work on a creature that has tentacles.
Giving an example of how you run something in your home campaign is fine. Adding rules and restrictions that don't exist as if it's written in the DMG somewhere? Expect people to challenge it.