Things like Haiku (or BeOS), ReactOS, ArcaOS, AmigaOS etc.

I don't think they do but maybe they're also counting type 1 hypervisors.

Edit: Nevermind, I thought they bundled in "Other" into that category; they didn't, as evident by the chart below the stats which includes "Other".

It's probably just the case of they couldn't determine the OS being used.