It's really quite simple. If taunting is on the books as a rule, then horns-down is a clearcut and obvious violation of the rule. There's zero gray area. Players throwing up the X with their arms has been called taunting. Players throat-slashing has been called taunting. Players putting the Texas Tech guns in holsters has been called taunting. And Texas players celebrating a TD with the horns-UP has been called taunting.
The taunting rule is stupid, but as long as it remains a rule, then horns-down is an obvious violation of it.
This is simple stuff.
If you don't like it, advocate for the removal of the taunting rules completely. I certainly do.