The Garleans have been stumbling across Allagan knowledge for a long time - long enough to know to bring the "lunar transmitter" to House Darnus long before the Bozja Citadel incident, and in that incident they decried the loss of a great amount of knowledge left by Allag (consolidated there to help Midas with his restorations). If the Garleans learned the word eikon from Allag, they must have learned what eikon meant, yes?
Perhaps the Garleans knew only abstractly of these entities and the effects they have on the land until Solus saw the Burn.