My guess is the clear plastic container that "shrunk and curled in agony" was PET, polyethylene terephthalate, recycle code #1. PET containers can indeed be very thick and sturdy .... and PET is fine for weak acids, weak alkalis, and water, as long as the material is cold or cool. Basically PET is great for iced tea, drinking water, and lemonade and not so good for anything else.
If a person does not want to use plastic containers for working with lye or soap batter, that's a reasonable choice to make. But I would hate to see advice given that implies stainless steel is the
only option for this purpose, because it is not. Sturdy containers made of polypropylene or polyethylene, recycle codes #2 and #5, are quite suitable and safe for working with concentrated NaOH, KOH, and soap batter. More:
https://classicbells.com/soap/lyeStorage.asp