Looked at the paper, answer: that's what happens when vulgarization of scientific discoveries is so bad that it removes the actual discovery from the paper. Apparently the author discovered the existence of memory t cells when writing that article.

You are saying that you learned about the mechanism of the cellular memory?

This study found that one way the pool is maintained for years after vaccination is through the development of several unique features. On the surface and through the actions of their genes, they look like cells that have never been exposed to an infection, but on their DNA the researchers found a fingerprint, called a methylation pattern, that identifies them as having been through battle as an infection-fighting cell, which are called effector cells.

No, you didn't. You may have learnt about immunological memory, but not the discovery this article is discussing.

I'm also confused but only because I've learned that cells have varying life spans and the only cells I know that last decades are brain cells.