Their 0-9 win over Southampton was no fluke
That final scoreline sure was a fluke 
No team makes it a habit to score 9 goals in a match, it has only happened a handful of time in history, so in a way it is a fluke for every team. But it is a reflection of the position a team is in and their ability to blow opponents away, the team has progressed so much since Brendan Rogers took over. They are among the top teams in points picked during that period and Vardy has scored more goals than any other player within the same period
Leicester showed big improvement this season. They're having a good start, currently at the 3rd spot but only 2 pts behind from Man City. Only two losses were outside against another top teams. Not saying Man United is a top team this early season but they are quite strong at their home games. The season is long and Leicester might lose some of its players, hopefully not. Giant clubs from different leagues are keeping their eyes.