Fact or fiction - have I ruined the chance to teach my cockatiel to talk?

I have a young male cockatiel. He’s about 7-8 months old. He mimics all sorts of whistles already like the wolf whistle, pop goes the weasel, charge!, and part of Darth Vader’s entrance song, he even whistles along while I play the piano (not always the right notes, but sings part of chops when I sit at the piano bench before I even begin…) He doesn’t mimic ANY words I repeat regularly to him, hi bird, pretty bird, his name- Joey, goodnight, kiss kiss, etc. His crest goes up and he cocks his head, so I know he’s listening and is interested. And I know it’s easiest for them to learn two syllable words, but he won’t. It sounds like he’s trying. Sometimes it sounds like muttering, almost like the adults in the Charlie Brown cartoon.

A long time ago I was told if you teach your cockatiel how to whistle that they will most likely not learn to talk, that they’re more content with what’s easiest and THAT will become they’re way to communicate with you. Has anyone heard of this? Is this true? Any insight or advice helps. I cannot find any online info supporting this either way about cockatiels. I’ve even found sites saying they will never talk which I know isn’t true, so I’m stumped. Why won’t he mimic words and phrases?