> "Truth is hard, let's go Postmodern!" isn't a valid conclusion. It's an escape hatch, a refusal to make a best effort and, when it's about something that matters, a horribly dangerous practice which essentially cedes the field to the idiots who have no such scruples.
Postmodernism is not the only critique of Kant. Although you're fighting a strawman, I agree with your criticism of postmodernism as used today, but that is not what I was talking about.
I am arguing indeed for making the best effort and especially about something that really matters. And it turns out "this is the objective truth" is one of the other escape hatches, especially since most of the questions that really matter are far more complex than the shape of our planet. Ethics, meaning, policy etc. they all require an ongoing process of being less wrong, which best works in opponent processing. There is rarely a terminal value for truth for these questions, there is always going to be an ongoing tension because the part of the reality we are trying to capture is inexhaustably complex. People who wrote the religious texts thought they were able to capture and codify the algorithm for "the right way", but we now see how inadequate they can be for our 21st century context. Anytime someone thought they had the key algorithm for utopia, they turned out to be the most evil machineries of our history sooner or later.
> But if you think that thinking the earth is spherical is just as wrong as thinking the earth is flat, then your view is wronger than both of them put together."
I think that guy is the most wrong. Because "what are we going to use this information for, what adjacent information that we haven't yet might be interesting, are we framing the question right" is the most critical parts of the query that goes unasked. If we are talking about walking on the street, the flat earth guy is just as correct as the sphere guy. Notice I am not saying his epistemology is correct, nor he is entitled to his own opinion and we should be pluralistic. I am saying as long as both guys only walk on the street, the difference opinions are completely inconsequential.
Now expand this to a question no one has a clear answer for. E.g. economic policy; some want it sphere, some want it flat. No doubt newspapers will be filled with people "it is flat, this is the objective truth". That is the use of going "hang on a second, we don't do objective truth here, we humbly pursue being less wrong, now dialogue with us, don't rhetorize your talking points, just honestly talk to us". Sadly, nowadays mass of the public discourse happens over channels that doesn't reward that but rhetoric and likes.