FAs can be really strange about that kind of stuff, not just out of ignorance.
My kid liked to suction cup his GoPro to the window to take a time lapse movie of the flight and one FA told him he had to take it off the window because he was, and I quote: "modifying the structure of the aircraft and that's not FAA-approved".
There has been a lot of debate in the aviation maintenance community regarding the legality of attaching gopros etc. to aircraft with suction cups. Someone eventually wrote to the FAA chief counsel and asked.
"Another consideration, in the case of this type of equipment, is the applicability of the term "alteration". FAA Order 8110.3 7E, defines an alteration as "a modification of an aircraft from one sound state to another sound state". The use of suction cups, or other temporary methods of attachment (not including permanent mechanical attachments to the aircraft), would not be considered a modification to the aircraft."
That memo is about attaching it externally. Attaching it to an internal window is probably a non-issue.
I once had a security agent ask me to prove a GoPro was a camera because they didn't understand how there could be no screen or viewfinder. It was most frustrating because this was an area where they would have encountered it many times (lots of scuba divers).
I would guess that the flight attendant is doing their job. They do not have the authority or expertise to risk the airplane based on their own analysis, or based on some random passenger's explanation. The clearly correct solution is to remove the device and then there is no risk to the plane. I expect they are strictly required to respond that way and have no leeway.
Had this happen to me with some duct tape and a malfunctioning strobing light next to me on a red-eye. I'm an aircraft builder but she didn't want to hear my explanation about how TSOs and the FARs work. I just waited until they stopped paying attention.
My kid liked to suction cup his GoPro to the window to take a time lapse movie of the flight and one FA told him he had to take it off the window because he was, and I quote: "modifying the structure of the aircraft and that's not FAA-approved".