It's possible to be right about the universe and the galaxy but wrong about the earth. The 90% that is accurate is still accurate even if another 10% is wrong.

More pointedly - we are all in the same position as the flat-earther you posit. None of us have 100% accurate knowledge - meaning some percentage of what we believe is either false or one day will be proven false. We all have blind spots. Flat earthers have a particularly egregious blind spot. So we can be skeptical of other claims they make. But we should still engage with the stuff that makes sense and coheres. The answer to bad science is more science; the answer to bad speech is more speech.