If you look at the page with $OS it looks like there is only a version for $OS available. I can see how that might lead to headlines like this. I also initially assumed it was a mac-only app.
In Chrome on Linux it reads "Opera Neon concept browser for Windows"
... with no mention of Mac. So the point is still valid.
If there was a version for Minix, now that would be newsworthy!
On Linux, it says for Windows too, with no mention of the Mac version (and presumably no Linux version exists)
I guess they just have an `echo (if os == mac then "Mac" else "Windows")`