I'm trying to imagine what kind of unsinger can consistently hit within a semitone but can't tune in to a reference pitch. Have you ever actually worked with someone like this?

When you sing into a tuner, the needle goes all over the place. You literally have no control over what it does. It's a complete waste of time. I'll bet there are very few people on this forum, no matter how skilled, who can hit a note and then consistently move their pitch up or down a few cents by watching the needle.