And Google wouldn't want to be caught telling lies...
If it's false, Google are in trouble. At the least, they'd be stupid to make this claim if there wasn't some evidence to support it. It's possible that Viacom outsourced their PR, and didn't authorise the posting to YouTube. But would a PR firm in that business be so reckless?
That's one of the reasons for having a trial. Both sides make claims, and present evidence, and challenge the other side's evidence.
Viral marketing, of course. A PR firm faking a grass-roots enthusiasm. It's sometimes called "astro-turfing", and is common in politics. It's the product of professional blog-comment spamming. Google's claim is very plausible. Get this into the courts and the implications could be interesting. An "anonymous coward" label here might still be OK, but I post some stuff under a nickname--what might come out of a case this big, involving material posted under false names?