DMG 47 states,
Only the severing of a character's silver cord while on the Astral Plane (or the death of his or her helpless physical body on the Material Plane) can result in the character's true death.
MM 160 states,
On a critical hit against a target in an astral body (as with the astral projection spell), the githyanki can cut the silvery cord that tethers the target to its material body, instead of dealing damage.
Does this mean that if the githyanki deals a critical hit against an astral traveler, it can choose to instantly kill its target?
That's what the feature seems to imply, but given that the githyanki has a ~10% chance of rolling a 20 on at least one of its two attacks per round, that makes fighting githyanki on the astral plane exceedingly deadly. Am I reading this correctly?