It's a deliberate decision to convey immediacy and urgency. Using present tense makes the memories read as closer and still unknown territory instead of farther removed and already determined, which matches the emotions of the memories that the author is trying to convey.

Consider the emotional difference between:

> It is summer, and the ice cream is melting.

> It was summer, and the ice cream melted.

These types of literary choices are a core part of writing prose at an advanced level the same way that camera angles and color palettes are a part of cinematography. Grammar "rules" don't really exist in creative prose, it's ultimately about using words to convey meaning however you can.

I don't think any grammar rules were broken. The author is setting a scene as if it were the present; think of it more like fiction, even though it's nonfiction. A narrator in a fictional story might say "the sun sets over the plains", even though that isn't literally happening in the present tense from the reader's perspective.

That's established literary style and not just in English.

The French language even goes as far as to have special grammatical tenses for different literary uses:

https://www.thoughtco.com/french-literary-tenses-1368875