But you can have an opinion on an objectively verifiable fact.

You can have opinions about objectively verifiable facts. And you can also have opinions on fiction.

For example, it is raining outside and I say, "In my opinion it is not raining outside."

That's not an opinion. And the assertion that it's not raining would be false.

Beyond that I think we would be twisting ourselves up in technicalities because when someone uses 'opinion' the way that this article refers to it, what they are really saying is 'I know' or the state of 'knowing'.

But that's not an opinion. Period. Just because you call it an opinion doesn't mean it is. Which is my point. Opinions objectively cannot be wrong. Do keep in mind my comment was more about the tone set by the original title, rather than the edited one (apparently it changed since I commented). Here's the original title for reference:

Yes, Your Opinion Can Be Wrong

As you can see, it's making an objectively incorrect statement. The new title ("No, it's not your opinion. You're just wrong") is much better and more accurately reflects what the article is talking about.

Hidden beneath this discussion is the real question: how do we know, what we claim to know? or more concisely: epistemology

Not really relevant, IMO.