Throwing the cup back at the server was too far and she should probably be banned from all Starbucks stores. But she sounded like she was mad that they used sugar instead of a sweetener. As a diabetic, it pisses me off when I order a drink and specifically mention the diet/sweetener option and servers ignore that and give me normal the normal soda/drink. It's not a fancy 'Karen' health choice - I'm not being pretentious. That shit could literally kill me. I'd be pissed off too if someone specifically ignored my request. It should lead to criminal consequences for endangering someone's life. My point is, you don't know the context so perhaps don't bring out the pitchforks over such a short video?
Pro tip for fellow diabetics - if you're not sure your drink contains sugar, use a drop of it and test it with your glucose meter. It can still read the glucose content.