While we don't normally think of them as such, most coaxials are horns! The horn is the mid/woofer/midwoofer's cone. No it doesn't provide the kind of horn loading needed for uber-efficiency; it's more like a "waveguide-style" horn, wherein pattern control rather than acoustic amplification is its raison d'etre.
So imo one possible implication is that THAT type of horn can be pretty good on vocals, albeit at the expense of some efficiency and bandwidth.
"Speaker design is tradeoffs, Highness. Anyone who says differently is in marketing." - from The Princess Bride (poetic license invoked)
Yes, but compared to a "real" horn, it lacks the issues that make horns sound less clear on vocals. Calling it a horn is not quite accurate imo, but no big deal, it's just semantics.
That said, I also agree with your last statement and I don't think coax drivers are as good as a separate tweeter. My impressions are that KEF/TAD/AJ type coax mid/tweets don't make for a speaker system that disappears in the room as completely as some other speakers. But the quality of the drivers allow them to resolve detail that can make vocals sound stunning.