Right, the 250 microgram figure was the maximum amount that you might dump into your system in one go, normal DMT blood concentration is far lower. The bare minimum for a psychedelic experience is 20k micrograms, but many people won't notice anything overt until 35k+, and a full "breakthrough" experience requires 50k or more, generally speaking, independent of weight.

For such a tiny amount of DMT to have a significant impact, it would have to be 40 to 100 times more "effective" than usual, or be supported by the soup of other chemicals released in those situations in a sort of entourage effect, with MAO metabolism reduced, and all sorts of neurons firing that otherwise would be silent.

NDEs often overlap real events, where full-on DMT trips shut out the world, so the entourage effect theory makes the most sense to me. Your brain gets overwhelmed, and the dump of DMT all at once, with serotonin, dopamine, and adrenaline maxed out as well, contributes to a predictable psychedelic effect on subjective experience.