The holes are in the assumptions of 10km/s delta-V, 1000kg craft and 1000km of tether. Make it a 100kg craft using 100km of tether and a much smaller delta-V leads to a much different result. A 1km/s delta-V is nothing to sneeze at. Additionally, half of that energy is transferred into the velocity and angular momentum of the captured object.
It's certainly a better idea than mine: wrapping the probe in a loose wad of double sided sticky tape and aiming directly at the comet.