I'm not sure where these people get the idea that some kind of thrust via the expulsion of mass is a requirement for moving and object. We know that gravity isn't really a force but an acceleration from the curvature of spacetime. The only way we currently know how to warp spacetime is with mass. There is no contradiction of conservation of momentum with our understanding that acceleration from the curvature of spacetime is a thing.

So, if there's some efficient way to manipulate the fabric of spacetime then it would allow for a propulsion method that doesn't need the ejection of mass. Far fetched? Absolutely. Possible that it might be discovered, it's possible.

Reactionless drives[1] have been investigated in other ways. James Woodward[2] proposed something that at least hasn't yet been debunked.

[1] https://en.wikipedia.org/wiki/Reactionless_drive

[2] https://en.wikipedia.org/wiki/Woodward_effect