FCTM 3.29 has this paragraph under the heading "Go/Stop Decision Near V1":
"It is important to note that the majority of past RTO accidents were not the result of an RTO initiated because of an engine failure. Full takeoff thrust from all
engines was available. With normal takeoff thrust, the airplane should easily reach a height of 150 feet over the end of the runway, and the pilot has the full length of the runway to stop the airplane if an air turnback is required."
My only guess is Boeing calls for an air turnback following an engine failure, hence the flaps 15.
I will have to check the QRH and FCOM but what you have reference isn't written in mine.