I think your answer exemplifies the chicken-and-egg nature of the problem....
I mean the true information is easily available - in this particular case for example it has been known for 2000 years how anybody can themselves measure the curvature of Earth.
...for example, if I'm a flat earther I can just go 'Who decides 'what is known' or the correct way to measure? You're just a tool of big geodesy' etc.
So why should i come upon them and militantly refute their freely chosen beliefs?
Well, if we were talking about going to the flat earth conference and mocking people, I'd agree. But we're actually talking about people who inject themselves into discussions and spread lies. That's equivalent to you and me having a normal conversation about satellites orbiting the (round) earth or whatever, and a flat-earther butting in saying 'satellites are a lie, the earth is flat you simpletons.'
When we're talking about misinformation/disinformation, we are not discussing person(s) who just happen to hold mistaken ro easily disprovable beliefs. I am OK with people believing in superstitions, religious ideas, or counterfactuals, in a general way. they may be at an earlier stage of knowledge, or the belief may serve some valuable psychological purpose for them, or maybe my belief is incomplete/incorrect and theirs is a better metaphor for the underlying reality, even if it is not fully worked out. When I run into people like and they solicit my agreement I might politely demur or express a different point of view, but I am not concerned to convert them to my own.
What I am talking about is the situation where people with such ideas join an ongoing discussion specifically to attack the beliefs of others, then claim they're being victimized by having their attacks refuted. Let's take a more current example:
Alice and bob are discussing vaccines. Alice says she just got hers and bob expresses the hope that everyone gets vaccinated. Carol joins the conversation and expresses doubt about vaccines.
Now, if that doubt is framed like 'I'm not so sure, have they really tested them long enough, or is [disease] really that big of a problem?' that is (probably) a good-faith doubt that can be amicably discussed and ideally agreed upon. But if Carol comes in with 'you idiots are pawns of the media's 5g conspiracy, wake up sheeple' that's not a good faith engagement. In such a case, Carol is probably not even attempting to persuade Bob and Alice, but to use them as props to create doubt in the larger number of people who read the conversation but don't participate.
Some conspiracy theorists are genuinely ignorant, or paranoid int he clinical sense. Where they are very insistent on promoting their wrong views, we can call that misinformation, as in mistake. But quite a few people deliberately spread untruths to stymie discussion of a particular issue or to damage the cohesion of a social community by lowering the signal:noise ratio of community discussions, and that's disinformation, as in distrust.