We do know that whatever Maiar are, they can have children with elves, from Melian and Thingol. Deducing that maia = elf from that is quite a leap in logic.
EDIT: Author was referring to Gandalf being mistaken for an elf, as per this excerpt of the unfinished tales:
"Mostly he journeyed tirelessly on foot, leaning on a staff, and so he was called among Men of the North Gandalf 'the Elf of the Wand'. For they deemed him (though in error) to be of Elven-kind, since he would at times work wonders among them."