Hacker News Reader

iOS is OS X. I'd bet it's someone who either didn't realize that the two popular mobile OS's are variants of common desktops, or someone who is playing with something like BeOS or Plan 9 or some other more obscure OS.

Could be someone who is writing their own.

Android is not really a variant of a desktop, it just uses the kernel, which is not desktop specific in any way, and everything above (from the libc up) is either custom or non-desktop specific.

Would you consider ReactOS Windows... or Linux?

Haiku https://www.haiku-os.org/