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Pedantic Grognard
No, they weren't. FR was explicitly Great Wheel through the end of AD&D, then was explicitly World Tree in 3rd/3.5, and then explicitly World Axis in 4th.The Forgotten Realms’ cosmological situation was pretty funky during 4e due to the spellplague, but my understanding was that the great wheel and the world tree were (and, well, still are, I guess) contemporary competing models in-universe.
There was fanon during 3rd that the World Tree was just an in-universe model, and that fanon got a bit extra popularity when the Sword Coast Adventurer's Guide didn't explicitly lay out which plane which FR god was on in 5th edition (the way the god-to-plane correspondences were explicitly laid out in previous editions). But the actual published material for 3rd (FRCS) and 3.5 (Player's Guide to Faerun) was entirely clear, especially the latter.