It's strange that the description text claims they died, but it's just a description.
The DOFF system seems to only have one state for DOFFs that don't return from any mission: Dead. So, whether they were transferred, used to build a colony (as this mission), or whatever, the status window records them as Dead if they didn't make it back to the ship.
Actually.... assignment outcomes have text that is different for each assignment. SOME colonist missions DO state that they are living happily in their new home(not a quote). This one obviously doesn't.