Ilúvatar was also the only name of God used in earlier versions — the name Eru first appeared in "The Annals of Aman", published in Morgoth's Ring, the tenth volume of The History of Middle-earth.
His climbing experience likely led him to write about the largest known redwoods like Lost Monarch in the Grove of Titans, or Iluvatar, described in that book along with delicate forest canopy ecosystems.