Is the reporter wrong, physics wrong, and/or am I just misunderstanding something?

I want to know how they determined the combined pressure of 640 African elephants. Were they all standing on each others’ backs?

You’re missing the explanation of how they do it, which isn’t really included in the article. It says that they are able to apply great force at low temperatures by some microscopic process that they hint may involve twisting in some sense, but they don’t make any real effort to say how in this article.

There is no mention of how long the process takes. Maybe they can create a very large pressure for a very short time. Just a guess.

Here is a better article written by one of the authors with a link to the original article.
https://phys.org/news/2020-11-diamonds-minutes-mimicking-asteroid-collision.html

From the article linked by @Caravanfan – “without added heat”

Only need 80 GPa (11.6 million psi).