I also recently read MN121, and Piya Tan’s helpful commentary on it. In MN121, there is frequent mention of “oneness” (ekatta) based on contemplating emptiness. For example:
Furthermore, a mendicant—ignoring the perception of people and the perception of wilderness—focuses on the oneness dependent on the perception of earth. Their mind becomes eager, confident, settled, and decided in that perception of earth.
This progresses through the arupa ayatanas and beyond to conclude with “focuses on the oneness dependent on the signless immersion of the heart” leading to nirvana. This seems to parallel the structure of MN137 (which as mentioned earlier outlines a sequence–starting with diversity, then unity and finally, non-identification) where awareness of oneness (ekatta) is part of the path, but ultimately discarded.