Reactionless thrust is less plausible, yes, but it is strictly impossible. Not all explanations for it involved reactionless thrust. NASA indicated that pilot wave theory might be involved (where the vacuum acts as the propellant). This research is pretty conclusive: it doesn't work. That being said, just because you don't have any reasonable explanation for an effect does not mean that it does not exist.
Flight would have been considered reactionless thrust before it was understood that air had substance.
Flight would have been considered reactionless thrust before it was understood that air had substance.