The food was on the plane. They refused to give it because she was in economy not because they didn't have it. Which leads me to believe that she could not have ordered food for the same reason.
She took the preventative steps of bringing snacks and hoped that the hot sandwich which was offered would be enough. It may have been on any other occasion. Unless security on American flights is different she couldn't take hot food with her.
When the flight staff refused she tried to get them to understand that her daughter's autism wasn't a choice. Considering that people in schools and hospitals don't understand autism when they bloody should, I doubt the staff on a plane would have seen anything more than a stroppy child. Then totally over reacted to what a meltdown could mean.
Now it is entirely possible that this mum was a mardy entitled cow. She may have set the whole thing up. However we can't know that.
At the very least the airline needs to train their staff better and allow more information to be included on booking eg hot food can be ordered in economy for disabled passengers.
I appreciate that this was in America but the equality act here would absolutely mean that they should have treated the girl more favourably eg give her a meal in economy when others couldn't have it. If the alternative meant she is treated less favourably eg chucked off the plane.