Heading back up to your logic up above–its not “units that can (in some situations) retreat”. It is units in this situation that can retreat (attacking tts yes defending ones no). That rule is written as it is to cover the situations where submerged subs, unhittable units (subs vs planes) or amphibious assaults where some units can retreat and others cant.
As a lawyer–I get what you’re trying to do and say that the rule doesn’t say what its supposed to say. As Krieg points out, that’s up for debate. Many lawsuits are about this kind of thing.
But that’s not because its dogma Canon law its because the other outcome is insensible, contradictory, gamey, requires a tortured reading of the language devised over multiple editions and revisons of the game to try and cover so many different situations.
there is no such thing as a perfect document try to write one