From the article: "During a dinner conversation at a gaming convention in 2012, Eric Harshbarger was asked by a board game designer if he could come up with dice that would determine who goes first — without the possibility of a tie. The idea was simple: settle the first turn quickly and get on with the game."

The original challenge was to design dice (I assume a single die would satisfy the requirement since the number of dice wasn't the point) that would quickly determine which of five players would go first. Nothing about the challenge required that there be five dice. A five-sided die would certainly be the simplest way, and would be marginally faster than having five players each roll a separate die and then compare the numbers.