Hypothetically, if, say, a Texas defender made a huge hit for a QB sack, and then stood up and did horns-up directly in the face of the opponent, then yes I'd consider that a taunt as well. But that's something so rare that I can't recall ever seeing it occur.
If that same player jumps up after sacking the opponent's QB and runs toward the sideline with the horns up, celebrating with his teammates, I would not consider it to be a taunt. Horns-up when performed by a Longhorn, in all cases I recall witnessing, is a positive celebration, same as a high-5 or a chest bump or any number of other generic celebrations. That it's school-specific to Texas, doesn't change its positive intent.
Horns-down on the other hand is ALWAYS intended to be a taunt, aimed directly at Texas. As far I I've seen, Sooner players don't throw the horns-down when they're playing Kansas State or Baylor (although some Sooner fans in the stands do, regardless of opponent, which seems weird to me). So it can't be considered a general celebration, if it's only done when playing Texas. It is always contextually intended as a taunt. Always.
Which is why I consider horns-down and the throat slash to be equivalent.
Still, if a Sooner player is running toward his own sideline or his own teammates and away from Texas players whilst doing it, I wouldn't consider it worthy of throwing a flag for taunting.
And as I previously said, I'm all for removing the taunting rules altogether.
But as long as there are rules against it, then horns-down equals taunting and should be called the same as any other taunt.
