Flex Queue Matchmaking

So I was playing flex with my buddies, our rankings are around high gold, but we get matched with a challenger flex player. How is this possible no one else was around that ranking. He is diamond in solo, challenger in flex, and platinum in flex 3s. Here's the match: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2461291538/38716001?tab=overview He is the kog "MadLyfe"