Soddim, I don't recall anyone saying that the aircraft in question actually did anything at the altitudes mentiones, although I'm certain it wouldn't have been too difficult to launch a missile while up there, and as the tests were conducted to assess if and how a satellite could be intercepted, I'm not sure they would need to do much while up there.
Last I heard, satellites weren't very agile.
Although I have no proof, this is a story I have heard several times over the years, the first time being from my wife who served, for a time, at RAF Binbrook, then seperately from various different sources, all who had served for much longer than myself.
Even if it IS an exaggeration (which I doubt), then it is one which find most enjoyable.
I'm sure you'll find lots of examples of aircrew doing things with their aircraft that designers say is impossible, especially with older, "designed" aircraft, rather than the new "computer generated" aircraft we see today.