Better is "negative energy gradient", or " inverted spacial curvature".
Antimatter and antimass are most definitely not the same thing.
Antimatter has opposite charge sign, but interacts with the higgs field in exactly the same way.
Antimass, is what you get when instead of tight coupling to the field, the field wants to exclude it.
The bad visual analogy, is if normal mass makes a "dent" on spacetime, antimass makes a "hill."
Now, the deal with prior talk:
You don't need actual antimassed exotic matter, if you can selectively reinforce a pilot wave that can only be ridden by "virtual" particles with that property. A virtual particle is an excitation state, that is not self sustaining.
Bad visual analogy: when you wiggle a string, if forms a sinusoid wave. Part of the wave dips down, and part arches up. The combined dip and arch can be thought of as a pair of antiparticles. The excitation of the string manifests these artifacts. The bohm interpretation says that particles ride such "pilot waves", following some rules, which exclude particles riding on certain parts of the wave. Since part of the wave is favorable to " antimass" type interaction (strongly excludes normal mass), and the other part is favorable to a normal mass type interaction (strongly excludes antimass), and the wave is subject to self interference from reflections, etc, effective pockets of antimass terms can be herded together in one region, and the normal mass terms on the other, and you get something like the alcubiere metric without the need for real antimassed particles. Virtual ones will work fine.
It is important to realize that these are Not "real" though. They are just the manifestation of certain parts of the excitation. Real particles are persistent excitations, that don't need additional energy to stay around. (According to Copenhagen.) These antimass terms will collapse with the virtual mass terms when you stop jiggling the rope.
No real particle with an antimass term has been experimentally observed. I doubt that they would be stable excitations in our universe.