Post History

The term "Linux" means the kernel but is often used as shorthand for
"Unix-style operating system running on Linux (the kernel)".

The reason this is significant is that the kernel is by far the most
complex part of the OS.  Writing one is a multi-year or multi-decade
project.

Unix-style OSs (ca 1990) were typically a kernel and a bunch of small
programs that interconnected easily using kernel services (the
"userland").  Each of these programs was pretty easy to rewrite and
maintain so most of them already had free implementations available.
But it wasn't until Linus Torvalds released an initial (semi-)working
kernel that the whole thing could be used as a FOSS operating system
on PCs.

(As to the GNU/Linux thing, yes, a lot of the userland came from the
GNU project, as did the development tools.  But even in the mid-90s,
enough people were annoyed at Stallman's GNU/Linux thing that it was a
truism that everything GNU could be replaced.  Today, there are
a few GNU-less Linux distros around.)