Right, the problem is that I said it was false, not that you said something totally false with such certainty.
I'm not sure how "I think..." conveys certainty.
It's just an abrasive thing to do running around going "False!" and "Wrong!" on the Internet. So yeah I'll give you flak for it.
I put in effort to try to not be abrasive because people call that out when I do. You're just receiving the same.
He didn't attack you or act abrasive. You are right that "I think" is not supremely certain language, but it does actually carry certainty with it since you're not saying "I don't know anything about this, but I think..."
Conclusion: I believe you're being a bit overly defensive, and should relax a little. He wasn't personally attacking you, he was saying you were false when you were objectively false, without much in the way of personal insult or even arrogance, and cited his source reasonably well.