That's not what "tuning" means. Yes, fretless string instruments, the human voice, instruments like the trombone are all capable of being played with arbitrary

That's not what "tuning" means. Yes, fretless string instruments, the human voice, instruments like the trombone are all capable of being played with arbitrary or semi-arbitrary pitch. But they are still either tuned in some way, or never tuned at all. Performance is not tuning.