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The mass of propellant being expelled is many times higher than the mass of air colliding with the rocket.

If that is the case then yes, agree, but there was no indication that is the case. The air is very thin out there.

In fact, it's linearly proportional, because momentum us m*v, so for 20 km/s propellant exhaust seed and 8 km/s air it needs to spend exactly 8/20=0.4 times the propellant to not slow down.

I understood that they want to used scoop up air to use as propellant. Only a fraction of the air it encounters will be able to be sucked in by the rocket.

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