I had a work-study job in college at our small local airport working for the FAA. I mostly clipped new FAA communications into 3-ring binders and tore off expired ticker tape weather reports for a couple of hours a day.
My boss was old school government employee, straight and square. One day we were talking and he handed me a FAA business card with a UFO reporting 800 phone number. Odd I thought, and put it in my wallet.
Years later I lost my wallet in a phone booth (am old). I got a call from someone who found my wallet and wanted to return it. He came over with the wallet which had everything still in it, including cash. The only thing missing was the UFO card. I've never seen another one.
Depends on the kind of flak gun, but you're probably right about the height being out of range there. i didn't realize they were floating the things over from that high up.
And yeah, likely safer to use missiles in that situation, especially since I'm not even sure how reliable those guns are at that kind of altitude.
It's actually a really curious military scenario too. Using reasonably old-school tech that probably outranges a lot of ground to air weapons, outranges drones, can be totally unmanned, and costs so little to produce and deploy.