@LordTrollbias
Personally, I'm more a fan of the theory that all of them are people who could have became legends. Those that did legendary feats but failed in the end to achieve their true goal so they were not acknowledged as heroes in the eyes of the rest of humanity.
Captain Ahab - Hunted Moby Dick, one of the divine beasts who reign at the peak of the magical monster hierarchy. Described as Nasu to be
"more super weaponish" than a living being. Even though he failed in the end, he still managed to do a number on such a creature while coming from a time where the great magics and heroes have waned. Heroes from the time of the gods probably couldn't have taken down Moby Dick but he still spent his life hunting it before getting killed by it.
Icarus - Although he died in the end while trying to reach the sun, he still managed flight which is a pretty damn big deal, especially in an age of mythology where the sun and the skies are territories of the divine (kinda like Tesla and lightning).
Asclepius - Although he became so skilled in healing that he was able to revive the head, he was killed by the gods as a result.
Amelia Earhart - Tried to be the first person to fly around the world. Which is probably a lot bigger deal in Fate than in our world (once again, see how Tesla harnessing the power of lightning which used to be a divine mystery gave him pioneer of the stars EX). She made it probably around 80 to 90% of the way there before disappearing.
Not sure about the other two since we don't know them though.