This confused me too, until I realized that the article is about pnpm, not npm (pnpm reads .npmrc for some reason, despite not having the same options as npm)
On a related note, it seems to be impossible to find the documentation of min-release-age by googling it. Very annoying.
I just set this up for npm, here's the command that worked for me:
npm config set min-release-age 7
The '7' is days. This is the only format that worked for me, just a single integer number of days.
Confirmed by trying to install the latest version of React 19.2.6 (published 5 days ago as of the time of this comment). It failed with a comment confirming that it could not find such a version published before a week ago.