Given that cause of death is not granted to Cabal and rather is considered inconclusive, I don't think this was intended. Phalanx punches send the physics engine into a whack and you end up dying from collision damage, often in entertaining and hilarious ways. Fixing it would be as simple as removing the knock-back from the Phalanx melee. Either it's been overlooked due to triage, or Bungie might just think it's funny and think it might as well be a one-hit anyway. Also keep in mind that since the cause of death is taking collision damage, if your flight path remains unobstructed you can survive by landing safely six miles away.