Maybe it didn’t fly at mach 5, but then it must not have done 3,500kts either. Just going by what others claimed it’s speed was.
Predictions are just that. Not a certainty. Maybe it will catch up at the right intersection, maybe not, Pilots have a funny way of being unpredictable in such a situation.
We call those sea stories. A bit of exageration improves the telling.
I used to teach SAM avoidance. The inability to fly over SAM sites is what ended the B-58, the B-70, and ultimately changed the B-1A into the B-1B.