Your comments about modern air-to-air missiles may be correct, but in 1968, the beyond-visual-range (BVR) engagement was more of a bomber general's fantasy than a reality. Kill probability of the AIM-7 Sparrow missile in those days was less than 10% [1].
Pilots' accounts of air combat in Vietnam routinely mention the firing of a salvo of missiles --rather than just one-- when a firing solution presented itself, due to the notorious unreliability of missiles.