Yes, even your wikipedia entry is clear :
Your quote says something about machine/network resources which automatically excludes applications because they simply aren't resources. Applications can be considered services that use resources. By targeting 1 or more resources you can cause a service to not be available any longer (because something doesn't respond and the entire thing hangs).
And that's the main reason why he's right and you're not: DoS/DDoS are targeted attacks at a certain resource(s) in order to make a service unavailable. This is not what's happening with this bug. You simply use a feature in the OS but it reacts in an unexpected way.
This bug can be used in a DoS/DDoS attack as you've said earlier but that's not what he was pointing out (read what he wrote carefully, I didn't see it the first time either). Although this is true it may not be a very realistic attack. It only really annoys a user, not an entire company. If you look at other attacks they are mostly aimed at retrieving data or at disrupting services to many users causing lots of loss of money. It's not a very attractive attack compared to others. Apple has already solved it in the newest 10.8.3 build so all in all I wouldn't be too worried about it.
If I send you a iMessage with File:/// as the contents, your iMessage application will crash. It's thus unavaible to you.
The OS X app hangs and will crash but the service will remain untouched. You'll still receive it on your other devices such as your iPhone, iPad, Mac running non-Mountain Lion, etc. Thus your conclusion is too simplistic. It is only valid for people who have only 1 device which is a Mac running OS X v10.8.2 (in other words: it is only valid in a very specific situation).
This is a bug that can result in a DoS exploit if you want to be a nitpicker.
That's why there is a definition of what a DoS is.