The chart and reasoning for the headline are horribly explained, but here's the line of reasoning:

ESPN currently makes $6/sub a month, and there are 100m subscribers, which leads to the $7.2 billion in revenue for ESPN from subscriptions.

Needham estimates that only 20m of the current subscribers would subscribe to ESPN. In order for ESPN to maintain current revenue, they would need to charge 5 times as much per subscriber per month, leading to the $30/mo number.

That wasn't the number they were referring to when they said it would cost $30 a month. They are saying that CURRENTLY every subscriber pays $6 a month, but since only 20 million of the current 100 million people paying for ESPN would pay for it a la carte, those 20 million would have to pay $30 a month to generate the same revenue.