As lumberjack pointed out, top lane is a massive slugfest which allows AoE damage to just ramp up. Look at how much magic damage she did.
20.6 K magic damage dealt to champions. You know what abilities she has that does magic damage? Just her W when it cancels dashes and that team didn't have many.
You know what does magic damage? Sunfire and Thornmail. You know what makes thornmail hurt more? A bunch of armor. She has 4 damn items to scale with thornmail, including her passive, and you all have three auto attack heavy champions that are building lots of damage. Even Hecarim went hybrid instead of a heavy durability build.
Combine that with how fed she is, the likely level gap between her and everyone else on the enemy team, the dragons and barons, and the ability to stay alive long because of Morganna's redemption, I'm not surprised she ended up doing a lot of damage.
Hell, just yesterday I played a game as full tank Sion where I had the second most damage dealt to champions.
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2382809032/212142416?tab=stats