They did not "rent" an AOC. The aircraft were operated as a series of charters. This type of arrangement is still quite common, although putting different variants or types on AOCs is time consuming and expensive.
By far the easiest way to start a "paper airline" is simply to charter another AOC holders fleet, or even a couple of aircraft.
Given todays climate I suspect most companies would want cash in advance and minimum contract period of a "season" .
Starting scheduled routes is a complicated process and by no means easy.