Hello chumbing Welcome to CF!
You guessed it! The actually 7pin data cable plugs in next to the 15pin power connector as seen in the 3rd photo there.
When following the power connectors coming from the supply look for the 15pin thin and wide connector like the one here.
You already found the 7pin data cable that plugs in next to it.
The opening with 8 bare pins is left as it is since that is only used for setting a specific mode such as seen in server cases. Your concern is only with the two data and power cables used for the typical desktop appication.