Whatever "revenue" they have is only to pay their expenses, including the cost of office space, salaries for their staff (not just the commissioner but his administrative staff), etc. That's how a non-profit organization works.
The NFL doesn't get any revenue from its football operations. My guess is that all of their income is from fees that are paid by each franchise.
The NFL blackmails cities into subsidizing stadiums for their billionaire owners.
They also insist on not paying any hotel taxes in the cities that host the Super Bowl.
If they don’t make a profit and don’t have any revenue over expenses, then they should not have to worry about being tax-exempt.