I think you're reading that implication in, when it wasn't there. Nobody can know with certainty what's really true or false, and the author isn't claiming to k

I think you're reading that implication in, when it wasn't there. Nobody can know with certainty what's really true or false, and the author isn't claiming to know either or judge people for disagreeing. We can only work on improving our processes for deciding what's true, and noticing that sometimes we can end up in a situation where our beliefs will become immune to contrary evidence is an important part of that. When that happens it doesn't mean those beliefs are certainly mistaken, but it does mean your fate is, more or less, sealed. If your faith in a flat earth is strong enough that you'll come up with new theories of optics to justify continuing to believe in it even after you're launched into orbit to see it firsthand, your belief is unshakable. But that doesn't guarantee that you're wrong, maybe you're a genius who just revolutionized cosmology and optics. But you're probably wrong.