The article says they have a lot of reserved instances which at some point is not fully used.
So if you have 50 cpu hours reserved capacity per hour and during parts of the day you're using only 10h, they can allocate the remaining 40h which they reserved and paid upfront for encoding.
So if you have 50 cpu hours reserved capacity per hour and during parts of the day you're using only 10h, they can allocate the remaining 40h which they reserved and paid upfront for encoding.