I hate that we're all ganging up on you but try to see it from a different approach...

Her job is first and foremost safety. If she was told for safety reasons all laptops over 2lbs must be stowed, she isn't going to waste time looking up things in a manual when she's trying to make sure X amount of passengers/cattle are following the rules and secure the cabin while a pilot is trying to land a giant hulking piece of metal carrying X amount of lives. Personally, I am happy that the FA was overly cautious in this scenario and valued the safety of everyone over one individual's customer experience.

After the flight is the time to nitpick over rules etc. Historically procedure has been a blanket "no laptops out during taxi/landing". . Now that laptops are made more lightweight that guideline might change, so it's best to clarify with Delta in writing, and I would point out the handbook to them.

I would be surprised if you found much sympathy here, but I wish you luck and hope your questions are resolved!