I'm not a doctor, but my understanding from talking to doctor friends is that the difference between a "Disorder" and a "quirk" is how much disruption it causes your life.
If when you see a plane flying low you hit the ground and start screaming, that's certainly a disorder. If you glance up at the plane and feel "hrm, I don't like it much", that's not really a disorder. In between those two extremes is found the difference between PTSD and "I don't like low-flying planes since 9/11".
I have about 3/4 of a panic attack if a big plane flies low near me in an urban environment. It has to be low enough for me to feel (or imagine that I feel) the vibration from the engines in my body.
Now that I think about it, I also get freaked out in moving crowds now, which I never did before. (I can be in crowded places, no problem--it's just when a lot of people are moving and mixing, like at a parade, music festival or busy shopping district.)
The airplane thing is totally manageable; I just need to stop and take a few deep breaths. The crowds thing starts off that way, but can get outright panic attack inducing if I don't get somewhere less crowded, and quickly.
I'm with OP. It seems that to be a 'disorder' it needs to have outwardly visible and functionally maladaptive outcomes on your behavior.
If when you see a plane flying low you hit the ground and start screaming, that's certainly a disorder. If you glance up at the plane and feel "hrm, I don't like it much", that's not really a disorder. In between those two extremes is found the difference between PTSD and "I don't like low-flying planes since 9/11".