I thought that comparing someone's belief on any subject, to that of someone who believes in a ''flat earth'', was just to ridicule their belief
as being synonymous with an outdated or ''primitive'' viewpoint.
I believe the person that you quote in your OP, was claiming that those that don't acknowledge ''chemtrails'' are of the same mindset as those
that believe in a flat earth.
This is quite possible, like I said I see it mostly used by one particular side of an argument.
But it can be used for both, as an example the myth of the flat earth was jumped on by Charles Darwin's supporters to discredit the church and to
paint them in an outdated light.
Really though calling someone a flatearther is just an attempt at character assassination, calling the person who disagrees with you a sheeple or
asleep or telling them they need to wake up is no different. We could all use a reminder to stick to statements and arguments with substance instead
of accusing our fellow ATSers of living in the dark ages