I don't think that's what he means. Some deaths would make it impossible to resurrect a companion - if the body wasn't intact (be it explosion or desintegration) or some death spells the characters would be dead - dead. I don't know if it's still in 5e. I am not terribly fond of those (failed save against this spell RELOAD) but I imagine it would be beneficial if we couldn't resurect our companions out of some situations - like vampire spawn sucking our companion dry.
When I say permadeath isn't a thing in D&D, it has an asterisk, because depending on the magic and resources available, death can be permanent
Without a body or body parts, the only way to revive someone is through True Resurrection, which is a level 9 spell, so out of reach for all but the most high level characters, and it's super-expensive.
Resurrection is required for severe body damage or dead longer than like 10 days, or for those turned into undead (I think).
Raise Dead needs an pretty intact body dead less than 10 days.
Revivify only works for 1 minute after death.
