Failure of astronomical proportions!
"Mars is quite close to the asteroid belt".
Sure, it's only farther from Ceres than Earth is from the Sun! The inner edge of the asteroid belt is about 45 million miles from Mars, about half the distance from the Earth to the Sun.
The boffins are clearly talking about "Mars crossing" asteroids, those that have strayed out of the main belt and come within, say, a few hundred thousand miles, just as there are "Earth crossing" asteroids that we hear so much about (Near Miss at 2 million miles!!).