Two things, though: First, sharp text embossment like that is perfectly doable in vacuforming with a female mold, and indeed that's how cups are made today. Second, the top lip curl, while hard to make out, wouldn't exist on a Styrofoam cup, and there's no information suggesting that Styrocup didn't also make plastic cups AFAIK.
Of course, that might not be a lip curl at all. It might be something else, such as a ridge, which was common on Styrofoam cups. But it looks curled to me.