Literally what? "Linux is good on the desktop because Apache isn't available for Windows" is a non-argument. End users don't care that Linux can run server software, and people who own servers don't care that Linux can run a desktop. The fact that both can use the same kernel, userspace, and package manager does not change the fact that there is a very real dichotomy. You might as well argue that MacOS is good for gaming because it can run productivity software just fine, and the latest Macs have GPUs that are (according to Apple's inscrutable benchmarks, anyway) as good as a midrange NVidia chip.

Authors of server software develop primarily for Linux. This is great, but not especially useful to desktop users, who have no use for server software, and who productivity software developers and game developers frequently ignore. None of that has ANYTHING to do with whether or not Linux is a "real" operating system. What Polar was trying to argue was that Linux is not viable for desktop use since it is rarely if ever considered by authors of software that desktop users actually need.