If the hit you take exceeds your total EHP and shields, you still get oneshot even if you have mesmer skin charges left.
This seems to also explain why he was getting oneshot by nullifies.
it's easy to reproduce. Just equip a decaying key and no suvivability mods and cast mesmer skin than go against a level 155 bombard int he simulcram.
Bombard test: https://youtu.be/2idGC5KVinw
Nullifier test: https://youtu.be/_ZM1RSxgEAY
it's even worse than I innitially