Totally understood, I think it suffers a bit from both a non-literal interpretation of the theme, and from the fact that a lot of the momentum mechanics are not obviously visible but are rather mechanics behind the scenes. When brainstorming the theme, I started thinking about idle games where you build up momentum by starting with small numbers and then snowballing, and then was thinking about arcade games like space invaders where the game builds momentum over the course of the play.
The player builds momentum with chained combo attacks, which directly multiplies your scoring, which in turn increases your strategic momentum by giving you more resources, and also reduces alien momentum. Aliens build momentum automatically over time, resulting in faster movement and attacks in the missions, as well as increased attacks and intensity in the strategic layer.