After landing, Shatner kept comparing the trip up and down again to life and death, as though life is a thin membrane of atmosphere that only takes a few seconds to go through, and then you’re dead, I guess. I don’t know why he saw it that way.
Sounds pretty accurate to me.
He was simply saying that the atmosphere and magnetic shielding generated by the Earth is an incredibly small layer that is completely transparent and delicate, yet, competently separates and protects all known life in the Universe from the dark, cold vacuum of space, which would mean death for all.
Even the light he was able to see permanently leaves the Earth, to never again provide its life-giving characteristic.