I think part of the confusion might stem from B. Thanissaro’s translation.
In that section of MN 118, the Buddha is describing how the progression of Mindfulness of Breathing which, with each in-breath and each out-breath, fulfills each of the Four Kinds of Mindfulness meditation (satipatthana: body, feelings, mind, principles). The Buddha says:
“Mindfulness of Breathing, when cultivated in this way, actually fulfills the Four Kinds of Mindfulness meditation [satipatthana].”
In explaining how it fulfills, when he gets to the second satipatthana, he says:
“Whenever a mendicant practices breathing while experiencing rapture, or experiencing bliss, or experiencing mental processes, or stilling mental processes—at that time they meditate observing an aspect of feelings—keen, aware, and mindful, rid of covetousness and displeasure for the world. For I say that careful application of mind to the in-breaths and out-breaths is an aspect of feelings. That’s why at that time a mendicant is meditating by observing an aspect of feelings—keen, aware, and mindful, rid of covetousness and displeasure for the world.”
Breaking down Bhikkhu Sujato’s translation of anapanassatisutta MN 118