This is the first time I felt that the title "Everything you need to know about..." actually was correct. I learned something and the text contained everything I ever might ever need to know about windsocks.
Is there some reading of this that makes sense with the figure being 180° instead of 360°?
But a windsock needs to be able to rotate over and over and over without bound. That's distinctly different from 360 degrees.
No, some of them can rotate their heads > 200 degrees in either direction. This gives them a full 360 field of view but they cannot rotate their vertebrae all the way round.
For a mechanical example, consider a common rc servo. They are specced as 180 degree, but from their normal center they can only rotate 90 degrees in either direction. Doesn't matter, they are still capable of 180 degrees of revolution.
So, your parent comment was correct, owls can rotate their heads through 1 full revolution. If they could rotate their heads one revolution either direction, then they could rotate their heads 720 degrees.
"Windsocks must rotate freely around a vertical shaft"
10 seconds google for "windsock rotation". Second result right after the original article here on HN. And arguably more informative
https://www.faa.gov/documentLibrary/media/Advisory_Circular/...