Does this mean that the trap is ignored?
Here is the effect.
When affixed to a horse, these horseshoes allow the horse to travel without actually touching the ground. The horse must still run above (always around 4 inches above) a roughly horizontal surface. This means that the horse can cross non-solid or unstable surfaces such as water and that it can move without leaving tracks on any sort of ground. The horse moves at its normal base land speed. All four shoes must be worn by the same animal for the magic to be effective.
So my horse can walk across friggen water, does this mean that it can walk over a pit trap? (I assume if you can walk over water, that means you're exerting zero pressure on the ground?)
I'd say no for this reason:
"The horse must still run above (always around 4 inches above) a roughly horizontal surface."
4 inches above, so if you run over a pit trap, suddenly that 4 inches is now 10 (or more feet down) and down you go with falling damage. Also don't assume 0 pressure to the ground as you're trying to apply real world logic to a magical item.
Uh, a pit trap HAS a surface...It's just designed to fall out when you put weight on it.
The magic items are created using the spell Levitate too if that matters.
"The horse must still run above (always around 4 inches above) a roughly horizontal surface."
It says a roughly horizontal surface, so as long as you have a surface to cross, whether it's stable or not, I'd say you should be able to do it safely.
I mainly look at it as how I've ruled in the past on it. the pit trap falls out, you go down. I also don't read it as the horse is rendered weightless but your mileage may vary. If you're DMing, go with what you want, if you're playing, just accept your DM's ruling.
I'd rule that the covering for a pit trap is simply an unstable surface. I'd allow it.