"The asteroid is falling towards Earth on a fixed trajectory, but the Earth itself is spinning beneath it, one revolution every 24 hours. This corresponds to around 1,000 miles an hour in the region of interest. So arriving ten minutes earlier or later would have placed the impact some 150 miles further to the East or West."
If the asteroid had arrived 10 minutes later on the same trajectory, the Earth would have moved about 18,000 km through space and it would have missed us entirely!
It's unlikely that the asteroid would have come so close (18,000 km is closer than GPS satellites) without being pulled to Earth because of the planet's gravitational field.
At 40,000 MPH (or about 18 kM/s) at best, the Earth could deflect it somewhat from 18,000 miles out. Remember that escape velocity is only 11 kM/s, so even if the asteroid grazed the atmosphere if it didn't slow by more than 7 km/S it would have just shot back out into space.
The GP is correct, if the asteroid is on an orbit that is outside of the ecliptic plane.
But to the article author's point, had it landed on nothing but ocean it would have had a different outcome. I believe the +/- 10 minutes conceptually was a way of illustrating some of the things that would have changed the outcome.
If the asteroid had arrived 10 minutes later on the same trajectory, the Earth would have moved about 18,000 km through space and it would have missed us entirely!