Because the linked article states that owls can rotate their heads 270°which is "almost 360°" according to the headline and post title.
When you see videos of owls this confusion becomes more apparent when owls don't stop rotating their head when they look right behind them.
They will often turn their head even further which suggests they can do a 360° turn, when in fact they cap out at 270°.
It's also noteworthy that there's situations where a persom could easily see and owl do a full 360 (and more) if the owl has it's head turned one way and then turns it the other.
270 in each direction for a total of 540 and all that.
Edit: I just double checked, and it's 270 degrees total, not each direction.