The Iliad isn’t old enough to rule it out. Apollo, in some ways the quintessential Greek god, was not Greek, he just resonated with them.
It carries a lot of clues that says it is certainly of Mycenaean origin. And Apollo's name has been found in Linear B.
I suppose we have to decide what we mean by ancient Greek though. If you mean pre-Mycenaean you are probably right. Importing him from Troy feels right. Importing him from Italy just does not jive.