That a DM can use some vague guidelines to jury-rig a disarm rule on the fly only makes it "untrue" in the sense that a DM can whip up a house rule whenever he feels like making a granny shot.
It's true in the fact that it's a basic application of the rules.
Otherwise, it's absolutely true that 4th edition rules do not make allowances for disarming someone without a power. If you don't believe me, go check out the last podcast David Noonan did with Mearls before he was let go. They say it's intentional that there's no disarming in 4e, and they proceed to explain their rationale for dropping the maneuver.
That's good for them. I'm the DM in my own game, though.
So, you've whipped up a way to go knocking the weapon out of an opponent's hand that's about as reliable as attacking them for damage. Are you comfortable with that? According to Mearls and Noonan, that's an undesirable thing, but I'm a little more optimistic. I'd be curious to know how else it works.
Yes, I'm comfortable with that. Now maybe I'd give a solo or elite monster a bonus to Defense - equal to their save bonus - but that's because I'm awesome.
A disarm can shut an opponent down completely, or it can be a meaningless trifle. There are other factors to figure out. Does the weapon just drop in the disarmee's square? And we all know what action it takes to pick up an object in your square, right? Or, does the weapon go flying off to some other square? It'd be good to know this before I try the maneuver. But, page 42 just doesn't help us out with any guidelines for this, true? So, it's a house rule or bust.
I would say it falls in the target's square, but this is covered by the player's declaration of intent and/or the DM's narration of effect.
I guess we disagree fundamentally on how the game is played.
Me:
Player declares an intent for his character.
DM decides if it's valid.
DM decides if it's opposed or not. (I'm including the environment as opposition.)
If it's opposed, we have to make a check of some kind.
If we are making a check, is it an attack? Then we decide what Defense is targetted. If not, then the DM provides a DC.
Roll.
If the roll is a success, your intent is achieved. If it was an attack, we figure out how much damage is done.
If the roll is a failure, the DM figures out what happens.
You:
If the books don't explicitly spell out how to do something, you can't do it.
edit: I will also keep it on topic and say that my game does not have the grind described elsewhere. If you want to interpret the rules in such a way that creates "copy-paste" tactics and combat grind, and you don't like copy-paste tactics and combat grind, maybe you should approach the game as I feel it was written.