It's MORE than Semantics.
Linux is not an OS. It never was. You can't download and install "Linux". You download an OS that uses the Linux Kernel (sometimes modified). BIG difference. There can be HUGE differences between OS distributions that use Linux as a Kernel.
Microsoft writes the Kernel to Windows AND the surrounding software that supports the Windows operating system. That's VERY different from the OS community that uses the Linux Kernel, or the OS community that uses the BSD userspace tools and libraries that they want (MacOS, OpenBSD, FreeBSD, NetBSD)
There is a reason Stallman refers to it as GNU/Linux, he is pointing out that Linus and crew are NOT responsible for the stuff surrounding the Kernel, and shouldn't get the praise or be damned for security vulnerabilities in things like standard C libraries, command line shells like BASH, etc.