[capeterson]

It could have something to do with the general idea of large numbers (e.g. 10,000) meaning a long time: https://en.wikipedia.org/wiki/Ten_thousand_years

As a result, even though 万 literally means 10,000, it is possible that 万 is used as more of a general idea for longevity. However, the article using the world "literally" would still be incorrect, I suppose!

Anyways, just figured that might clear up a small part of your question, since the usage of 万 is not necessarily literal in many East Asian languages.