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.
Last time I looked it, this EM Drive research was not NASA project. They just gave a NASA engineer a warehouse to pursue his ideas on his own time like good research organization does.
The "Eagleworks Laboratories" or "Advanced Propulsion Physics Laboratory" is just what Harold G. White calls the warehouse. It has no website or staff from NASA.
It was not "NASA" who indicated something, but a couple of guys working for NASA. Also, the "pilot wave theory" stuff is just idle speculation on topics of which the authors did not appear to understand much.
Flight would have been considered reactionless thrust before it was understood that air had substance.