Klas,
As always, I'll defer to your expertise. Yes, I am aware of the clause you cite. However, the last paragraph of Rule 3.2.3 refers to a broken unit that becomes adjacent to a KEU. It does not say "In the same hex." Although, one would think it would have the same effect.
I guess my confusion arises from the first sentences of 3.6, "During the RtPh a broken unit not in Melee may not end a RtPh adjacent to or in the same location as an unbroken Known Enemy Unit and may not remain unemplaced in the same Open Ground location in the normal range and LOS of a Known Good Order enemy unit that would be able to interdict it if it were routing in that hex (see below). Such a unit is placed under a DM counter and if not in Melee must rout away...'
From this I gather that a broken unit not in Melee may end a RtPh in the same hex as a broken KEU, and not have a to be marked DM.
Again, if we are talking about units (formerly) in a Melee stack, my thinking is they probably would have been broken as the result of a fire phase, and therefore would have been placed under DM already. Which perhaps makes it a moot question anyway.
I await your insight and thank you in advance.