It’s both. Some domains do not allow WHOIS privacy (.us is one of them), some have it built-in, while most don’t care and registrars can fill in with fake data.

They probably meant "whois privacy"[1] (without the space). Whois is basically a way to get information about a domain name (and many other stuff). Whois privacy just ensures that your address, name and other stuff is not public.

[1]https://en.wikipedia.org/wiki/Domain_privacy