> like every work of literature that characters reference is from the 20th c

A lot of literature today refers to bronze age religious texts(2700 years ago), Shakespeare(400 years ago), the Ramayana(2600-3kya), Romance of the Three Kingdoms(600ya), and so on. There are threads of Porto-Indoeuropean mythology preserved in the Vedas, Roman Mythology, and the Scandinavian Sagas.

I don't know much about Australian Aboriginal mythology, but there's strong evidence that their stories contain accurate details of extinct species and geographic features from thousands of years ago.

So it seems entirely possible that enough cultural material from the mid 20th century will survive until the 2500s for people at that time to have a good understanding of the 1950s in at least some places.

Not familiar with that particular story, but doesn't it just reflect the popular mid-late twentieth century scifi conceit that interstellar travel would start at the end of the twentieth century and be the most important thing ever? In which case the 1950/90 distinction probably would be a consideration for someone with a half decent knowledge of history, a bit like someone today saying "imagine telling someone in 1450 or even 1492 what the Americas would be like today..."

Minus points for thinking the literature would stand the test of time though :)

Reminds me of reading Asimov's books set in the distant future and they still use tapes and other things from the 1950's. Maybe if pressed he could argue that the technology in the future was vastly different, but wanted to use the name of a storage media his readers were familiar with and avoid having to make up a new word.