TMP makes the same implication with an impulse engine.
More than likely as warp engines started becoming more efficient they needed greater and greater magnitudes of high energy plasma. In TMP it's likely impulse plasma could power the warp coils. Likely this was a change in the materials the coils were made from.
With Fusion reaction there is ceiling to how much energy the plasma will have have but anti matter is a reaction that turns 100 percent of the matter into energy.
That's why to it makes more sense that the first warp engine was actually an impulse engine since warp and impulse essentially are both types of field propulsion.