Thank you to the xkcd April 1st volunteers/commentators, including @Chromakode, Kevin, @Aiiane, Patrick, Kat, Reuven, @cotrone, @bstaffin, @zigdon, schwal, Stereo, and everyone who voted!
This was the ninth April fools' comic released by Randall. The previous fools comic was 1975: Right Click from Sunday April 1, 2018. The next became 2288: Collector's Edition, which was delayed two days and released on Friday April 3, 2020. The interactive comic began at noon ET (16:00 UTC) on April 1, 2019, and ended a day later. In it, users were shown two emojis and voted for their favorite before the time ran out. 512 different emojis were paired against each other in a cup or bracket system, with only one winner. See more below under How it worked.
Brackets - like the one in this comic, for finding the best emoji - are a recurring theme in xkcd. It is also relevant for this time of year, and two years ago in 2017, the first comic in April, 1819: Sweet 16 from April 3rd was a bracket, referencing March Madness. The 2019 version of the National Collegiate Athletic Association College basketball national championship tournament began March 19th and ends April 8th 2019. So this comic could also be said to reference this, although it is not so explicit here. Earlier Randall made another large and "silly" bracket in 1529: Bracket (which someone then actually made into an online voting system, just like in this comic).
The title is a reference to the movie Mad Max Beyond Thunderdome, which had the tagline: "Two men enter. One man leaves." The "Thunderdome" in the film is a gladiatorial arena where conflicts are resolved by a duel to the death. In the final round, the Milky Way emoji (🌌) won against the Hedgehog emoji (🦔). The comic was updated to show the result.