No disagreement that less delta-V is needed for big rocks far out in their orbit, so early detection is cheaper.

... but in terms of whether stopping is possible, I believe that depends on the amount of kinetic energy you can apply to it. I'd have to sit down and do some real math to determine the practical stopping power of a kill fleet, but to give a thumbnail comparison: each Starlink sat is about 500 lbs, which is about the weight of one B57 nuclear bomb, with a yield of 5-20kt. There are about 660 Starlinks in orbit now... A constellation of kill vehicles of the same size could have the ability to deliver an upper-bound of 13.2 megatons of explosive energy. According to one source (https://www.space.com/39971-asteroid-destruction-nuclear-bom...), 3 megatons should be sufficient (though the parameters for individual delivery of smaller payloads vs. one big nuke would of course be different).

This is, of course, all a terrible idea; nobody's gonna be comfortable with a mega-constellation of nukes in orbit.