What we are looking at is what is referred to as normal and rational skepticism (which should be part of inquiry in any field, and what is referred to as "radical skepticism" which is based upon the fundamental assumption that it is impossible for the human being to know anything.  Those who are "radical skeptics" then have to replace that with something else, usually blind belief in one thing or other.  That is what not only the "flat earthers", but is characteristic of zealots in any cause.