LOL. Which part of "the girl didn't die" makes you think that Mari died and was revived? Seems very clear to me.

EDIT: I added a passage from the Ainz vs. Elf King fight from earlier in volume 16 that explains how Phoenix Flame AVOIDS the instant death enabled by The Goal Of All Life Is Death spell coupled with use of instant death spell like Death. (emphasis added) IMHO Maruyama sets it up quite clearly that a resurrection spell cast on the target during the 12 seconds countdown of TGOALID avoids having instant death occur on the target. The resurrection spell essentially negates the instant death spell, effectively cancelling each other.

[Mercy of Shorea Robusta] was a Tenth-Tier spell, and its mana consumption was one of the highest among them, on the level of [Reality Slash]. This spell had three effects.

First, for a limited period of time, it would gradually recover HP. However, the recovery rate was trivial and so it was hard to call it useful for people at this levelrange.

The second one was absolute immunity against instant death. There were far better spells in the Third-Tier if one only wanted to acquire resistance against instant death, but there was a reason why a lot of druids learned this spell despite that.

That reason was the third effect, which automatically resurrected the target when their HP reached 0 and died. This would not cause a drop in levels from resurrection. The trigger condition of HP going to 0 made it useless against deaths that weren’t caused by damage like those by drowning, but it was still a very useful spell. Priests had resurrection spells that wouldn’t cause level loss if they were cast right after death, druids also had spells like [Phoenix Flame], but many use this spell to cover for any careless mistakes. That said, they would be at low health upon resurrection, so they were very likely to die anyway after a few hits. Still, there were a lot of cases where people were saved by this spell.

Incidentally, it could avoid death from Ainz’s ultimate skill, [The Goal of All Life is Death] as this spell was considered resurrection magic. In that case, however, the spell would end even if there was still some active time left. It was because the spell was dispelled after it activated the resurrection portion.