Corsairs treat Eldar, Harlequins and DE as Battle Brothers, and cannot ally or be allied with Necron Chaos Demons or Tyranids. Not come the apocalypse, never can not. The following sentence states that they treat other forces as desperate allies and are treated as desperate allies for them.
If one where to circumvent the banned detachment rule such as through malefic spells, how would the units interact per the allies matrix?
Do they become desperate allies per the followup sentence?