Columbus
And thus was smallpox introduced into the previously Undying Lands.
And thus was smallpox introduced into the previously Undying Lands.
The comic starts with White Hat telling the two children shown on the first panel that Christopher Columbus knew the world was round, but that others believed it to be flat. However, this is a false narrative known as the Myth of the Flat Earth. Educated people in Columbus's time knew the world was round, and knew the approximate radius of the Earth. Columbus claimed that the distance to sail west from Canary Islands to Japan to be about 3,700 km, drastically lower than others believed, but he was wrong about this. If another continent and the "West Indies" had not been fortuitously in the right place, Columbus and his crew probably would have died at sea.
As White Hat begins his explanation, Megan objects, though not explaining why. White Hat continues, so Megan interrupts, saying that Columbus went in a straight line as the world curved away, ending up in Valinor and the Undying Lands. Megan's story is an allusion to The Silmarillion, by J. R. R. Tolkien, set in the same world as The Lord of the Rings and The Hobbit. The claim that Columbus sailed on a tangent to the surface alludes to how the elves' ships leave the curved sea surface and sail in a straight line to reach Valinor on the same route that they sailed when the world was still flat. The mentions of a silmaril and the morning star are a reference to Eärendil the Mariner, the only mortal sailor to reach the Undying Lands, with one of the Silmarils (though Eärendil's journey occurred at the end of the First Age and the world was only changed into a sphere near the end of the Second Age). Megan humorously conflates these myths, suggesting that they are all equally false. Columbus in fact wasn't the first to claim the world was round; the ancient Greeks had discovered it long before. It was, however, disputed by some Christian scholars in late antiquity due to disagreements over its congruence with biblical canon. In Megan's telling, Columbus ends up as the morning star, which is actually the planet Venus (the same fate as Eärendil's in Tolkien's mythology).
The joke is that when White Hat tells her to stop making up the story, Megan pointedly replies "You first", indicating that she originally complained about White Hat's retelling of the Columbus story because his account didn't really happen, and so he was also "making things up". Megan's fantasy tale was then delivered to make a point.
The title text refers to the transfer of smallpox to the Americas by Europeans, which caused the deaths of untold millions of Native Americans. The introduction of smallpox to the Undying Lands would indeed make their name ironic. However, the Undying Lands are named after immortal Valar, Maiar, and Elves living there, not because they confer immortality. A more proper name would be the Lands of the Undying, and Valar, Maiar, and Elves are not susceptible to diseases in Tolkien's mythos in any case.
Similar discussions between White Hat and Megan can be found in 1605: DNA and 1731: Wrong, in the latter Megan even finishes with a similar *sigh* as she started with here.