Unfortunately "core" can mean one of two things depending on who says it and in what context: (a) it's bundled with the PHP source and available as an extension, or (b) it's bundled with the source and baked-into PHP automatically. The former is more about the fact that it's officially maintained by the PHP "core devs", as opposed to being something third-party or in PECL, and the latter tends to mean the extension acts as a dependency for important behavior (eg, ext/tokenizer).

Thing is, this is all still subject to how distributors want to deal with it. Particularly Linux, which very often separates "core" stuff into optional packages to install. The Windows download is about as close as it gets to some sort of "default" PHP installation.

And with the Windows download, sodium is an extension. So it's "core" in the first sense.
On Windows you can get it by enabling that extension, and on Linux you'll probably have to install some package.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.