An aircraft with an upward vector does not need power to climb. TWA 800 was climbing at the moment of the explosion and then the nose of the aircraft pitched up, giving it even more upward momentum. A climb of 1700 feet is not that significant in an aircraft traveling with the momentum of a 747.
Where does the 1700 feet come from? The height gain was reported in the 3 - 4 thousand range. Even so, the angle change for 1700 or 3000 - 4000 feet is small compared to the 13000 feet initial altitude. Would hardly explain witnesses statements that object went upward in some cases from the surface.