It completely depends on what they call "a change", and the article does not make that clear.

If they're doing some sort of strict continuous integration, then, a "change" could be a 25 lines function with a 100 lines of unit tests, in the frame of a large project where the function will be used later in a UI component that will only be merged in two weeks.

The fact that it's "deployed" does not mean it's "used" in production as a final thing ; it might very much be "a step in the development".

And, even if they're shipping "feature" (that is, they're deploying the last commit in a project), it does not mean that all millions of users are seeing the change (they could use feature toggles, A/B testing, etc...)

Seen this way, about 2 PRs per day per eng is not unreasonable, and with enough devs, you can reach it.

Finally, they might very well have some automated PRs (i18n, etc..)