Clearly the controller was anticipating having plenty of separation on the next departure, and in fact, the delay was long enough for the pilot/controllers to notice at roughly the same time to abort the next takeoff. You wait an extra 15 seconds between every plane you are dropping a 60 departures an hour rate down to 48/hr, you will be running hour plus delays every day in no time.
This is correct. In fact, he was implementing 3-9-5 and 3-9-6 from the .65:
3-9-5. ANTICIPATING SEPARATION
Takeoff clearance needs not be withheld until prescribed separation exists if there is a reasonable assurance it will exist when the aircraft starts takeoff roll.
3-9-6. SAME RUNWAY SEPARATION
Separate a departing aircraft from a preceding departing or arriving aircraft using the same runway by ensuring that it does not begin takeoff roll until:
a. The other aircraft has departed and crossed the runway end or turned to avert any conflict. (See FIG 3-9-1.) If you can determine distances by reference to suitable landmarks, the other aircraft needs only be airborne if the following minimum distance exists between aircraft: (See FIG 3-9-2.)
1. When only Category I aircraft are involved- 3,000 feet.
2. When a Category I aircraft is preceded by a Category II aircraft- 3,000 feet.
3. When either the succeeding or both are Category II aircraft- 4,500 feet.
4. When either is a Category III aircraft- 6,000 feet.
In this case, the A320 and E170 are in the came category. so 6000ft would have insured enough separation to give RPA their takeoff clearance.