Horns are a natural weapon, and part of the character's astral form because of it, and so could certainly be used in astral combat. You'd still use Charisma instead of Strength though, and Astral Combat instead of Unarmed Combat. I do think that's stupid. I dunno why my adept, who's using her physical body to swing a weapon focus, has to use her Charisma and can't use her Blade skill, despite the fact that physical impact clearly matters (if it didn't, a Weapon Foci Combat Axe wouldn't do more damage than a Weapon Foci Combat Knife) and she's still physically swinging a bladed weapon to strike something... Well, I suppose if others thing that's also kind of unreasonable I can see what the errata team thinks.
But currently, in RAW, all (of the player's) attacks against astral beings use Astral Combat instead of their normal combat skill, and Charisma in place of Strength. Having horns wouldn't logically be an exception to the rules. But since weapon damage is a factor, the horns being used would improve your damage.
