Patient
Anaphylaxis doesn't come from nowhere, as you quite rightly pointed out.
It might come from an antigen, several hours previously, that the patient doesn't know that he's allergic to. It might come from some crumbs of peanut left on the aircraft seat, or some pollen left by a previous passenger, that again he has no knowledge of.
In hindsight, in this situation, after the guy recovered after landing, it sounds like it was a panic attack.
At the time the guy was taken ill, without the benefit of a crystal ball and prior knowledge that he was going to recover, such a diagnosis was impossible.
Not wishing to needlessly carry on a thread past its usefullness, and accepting that you have personal knowledge of panic attacks, but the diagnosis of panic attack is a dangerous one to make. Panic attacks in themselves and associated hyperventilation are physically harmless, although frightening (unless they open a door and jump out the aircraft!), unlike the alternaltive conditions that might be going on.
As a paramedic I would have to be very very certain that was all that was going on before leaving someone without taking them to hospital. Normally we'd take them in just to cover ourselves.
Anaphylaxis, heart attack, low blood sugar, stroke...etc etc are all things that certainly couldn't have been ruled out by the original poster whilst trying to fly an aircraft, or even by an experienced doctor travelling as a passenger alongside the sick guy.
Once again, LondonJ did the right thing.