I feel like you failed to read his post correctly because he's a Nov10, but it's possible you just don't understand physics-related words =.="
The drag force he was referring to means that the particles impede ships' movement, not let it happen. If there were literally no particles in space (and the universe could still exist without all matter exploding from diffusion–oh, wait, that already happened, and that's why there are particles in space), then a ship, once far enough out of notable gravitational fields, could be propelled once. And that would, eventually, make it travel an infinite distance at a constant rate.
That doesn't happen.
Or maybe English is not my main language therefore I don't understand some ENGLISH physics-related words.
I reread it and now I understand what he meant. For a moment I thought he was talking about propelling ships in space. But it's still true some parts of space are a complete vacuum. Not all of it of course. Therefore, said drag force is ridiculously low.
Oh and I didn't even notice his join date. Like I care…