I apologise. I had forgotten about the FAQ.
Independent Character IS a subtype but even if he had it the FAQ prevents him from joining units just like our Coldstar Commander.
All of the Tau HQs are "Infantry (Character)" or "Jet Pack Infantry (Character)." I have yet to see a datasheet in any current codex that has "[Unit Type] (Independent Character)." That's because Independent Character is a special rule, not a subtype. But this discussion is now moot because the rest of have been revealed, and as you say, Independent Character isn't among them.