Could you point out where the GP mentions flat earth theory? I'm not sure what your point is...
I think his point isn't necessarily flat earth, but the real reason people "feared the vast ocean just as much as we do space". They weren't at all afraid of water or being at sea -- during the time in question naval power was the dominant military power and every emergent imperial power did so through their navy. People didn't "fear" the vast ocean, they just didn't know other continents were out there, and so they quite rationally "feared" that if there was nothing but thousands of miles of water between Gibraltar and Canton, it would be extremely difficult to stock a ship sufficient to support all the crew over that long of a journey while still preserving room/mass for the cargo necessary to make the trip financially worthwhile.
So there's a significant difference here, in that we can actually see what's in space between us and our destination. And we don't see a damned thing that looks anything remotely equivalent to the luxuriously resource-rich American continents discovered by early European explorers like Columbus. When European ships pulled up to the New World they were encountering basic natural resources which have high utility for human beings with no special technology -- timber, fresh water, game, plant foods, arable land. And those two land masses are truly massive, stretching nearly from pole to pole across the surface of this planet. Space on the other hand, contains almost entirely, well, empty space. Hence the name, Space. Any matter/energy we do encounter is going to be in a form which will take significant technological infrastructure and on-the-spot engineering for us to have any use for it. We're not going to just accidentally stumble on a Kwik-e-mart with rechargeable batteries and slim jims between here and Proxima Centauri.
Therefore, the "fear" of the vast ocean in the 1400s is not at all equivalent to our "fear" of the hostility of dead empty space -- because we can directly verify that space does indeed consist of millions of light years of cold dead nothingness. There's no "there" there.