What's even worse than the shoddy probability argument, is that the argument is deliberately obfuscating the real issue. Even if you accept "Oh yeah, you're copyrighting numbers" as a legitimate argument, it's irrelevant to the issues around the HD-DVD key nonsense. No one is asserting copyright over the HD-DVD encryption key. The DMCA does not grant them any new right to copyright an encryption key - they always had the right to copyright it as an encryption key: but as such, it would be subject to constraints like fair use. Instead, what the DMCA has done is create something new. They don't need to assert that they have a copyright on the number, and thus have rights over its copying and distribution. In fact, they are not asserting that they have a copyright on that number. What the DMCA does is say by virtue of the fact that they used that number to encrypt some copyrighted work for the purpose of copy protection, that they have a greater right to control the use of that number than they would if they merely had a copyright.
Let me repeat that, because it's a critical point. They are not asserting that they have a copyright on the HD-DVD key. They are asserting far greater rights than what is granted by copyright. Under the DMCA, by virtue of its status as a copyright protection circumvention device, they have far more right to sue over its copying and distribution than you or I have to sue over copyright infringement of our creative works. They've created a new category of intellectual property - not copyright, not patent, not trademark. And this new category gives them an obscene degree of control over the use of that property - which is just numbers.
Go, read it!