Not necessarily. Bahamut himself was known to spar with his followers in his true dragon form to prove their worth. While it's highly unlikely, it's entirely possible to destroy him on his home plane. No god in the Faerunian pantheon is completely immortal or invincible, in fact a fair number are mortals ascended to godhood. Bhaal, Bane, and Myrkul were mortals who l killed a primordial god, then traveled to the domain of Jergal, the original god of death, to kill him. He instead offered his three portfolios to them, ascending them to godhood. Bhaal was later slain by the mortal Cyric who then took over his portfolios and ascended to godhood. Gods in Faerunian pantheon are not omnipotent or omniscient.