I'd be more interested as to what sort of action dying from hit point damage is, is it a free action, or is it an opportunity action?
If it's a free action, can you negate it with the stunned condition?
I'm curious as to how one can say the attack action is a distinct thing from the 'damage action'.
A lot of these rules arguments don't make a lick of sense, and I'm tempted to disregard them for being rediculous.
My guess is its not an action. The same as Delay, monster knowledge checks and (surpisingly) some utility powers are not an action either.
Edit - I think you can interrupt damage, you just can't negate it so the best way to look at it is that not every thing can be cancelled with an immediate interrupt.