I noticed you didn't bring up them also taking napkins, sauce packets, sugar and creamer, cutlery etc. You either already were eating at wealthy enough places (metal cutlery + real cloth napkins) that this wasn't an opportunity for you & that was itself a class indicator, or you were focused on one thing (taking food home) and missing the bigger picture.

Of course each individual action of preventing waste isn't a 100% indicator for having grown up in poverty. Low-waste is arguably fashionable, even. But being low-waste is different from acquiring and retaining arbitrary stuff to use later in place of things you can just buy when you need it. Buying a reusable straw is different from keeping every disposable straw you're given, same as buying a reusable bag is different from keeping every takeout/grocery plastic one. The house of my parents displays my impoverished childhood clearly-- it is a place of incredible resourcefulness using all the things people normally refuse to acquire or throw away if given to them.