You're wrong. He wasn't spouting this off as opinion but as fact. So that takes away any first amendment protection. It also hits two of the needed checkmarks, you only need one, out of the three required to prove a libel case. "To win a U.S. defamation lawsuit, the plaintiff, at the very least, must prove that the defendant:

Caused material harm to the plaintiff by publishing or broadcasting said false statement of fact;

Acted either negligently or with actual malice;"

I won't say it was done with malice. He thought he had the right guy. But he thought he had the right guy due to him acting negligently. DramaAlert claims to be reporting on news from the youtube community. But he wasn't posting claims from other sources. If he had then he'd be in the clear. Because saying "Bill said this guy is a pedophile" or "New York Times is saying this guy is a pedophile" he is just reporting what others are saying. But since he was making these claims himself and stating them as fact, and not opinion, he was negligent by not doing a bare minimum of due diligence. He didn't do any research and just spouted this information off as fact causing this man harm.

Whether the damages amounted to being worth suing over I don't know. He could get money for lost revenue, therapy sessions, etc. But he absolutely could have been sued for libel and more likely than not won his case. To circle back and finish. Keemstar believing it was true would have held up just fine if it came off as opinion and not as fact. He didn't say "I think this is the same guy". He said it was.