It sounds shocking because it's one sentence which delivers two surprises; Oxford is older than expected and the Aztec empire is more recent than expected.

I agree. It’s not a comment on the existence of a people group but rather a political structure. Saying something is older than the USA does not imply nobody lived in North America more than 250 years ago.

No his point is that this is surprising because people have the wrong idea of what the Aztec empire is.

It's like saying "the population of the City of London is only 8500 people!" - it's only surprising because it's misleading.