The problem with those links is that they're message boards, literally anyone can post a response to any thread, there's no authenticity to who can post what because anyone can say anything. You should go to the official pages created by pokemon sites rather than the message boards because the official pages are made by people who are involved in the site and therefore want to be as accurate as possible, as well as often they have actual access to the data of the game, whereas poster on the internet can simply just play the game and just assume things and post them as fact. So anyway, I always thought Trace copied the ability directly across from the user (like Imposter) but it actually copies the ability of the adjacent opponent. meaning that if you are in a triple battle and you are on the right, it will copy either the center or (from your perspective) the opponent on the right side of the screen (across from you) but will not copy the opponent (from your perspective) the opponent on the left side of the screen (opposite from you). Now, when it comes to horde battles, all 5 of the opponents count as adjacent to you (as is shown in the target select screen) so Trace literalyl can slect any 5 of them. The best way is to use an ability altering move (Worry Seed, Simple Beam, Role Play, Entrainment, Skill Swap, etc) I prefer Skill Swap as that is what my Gallade can learn (which also happens to be my capture pokemon with False Swipe, Hypnosis and Shadow Sneak for ghost pokemon and to have a move that actually KOs a pokemon) but yes, Trace can hit literally any random opponent in a horde encounter, not the one on the far left:
https://bulbapedia.bulbagarden.net/wiki/Trace_(Ability)
https://ca.ign.com/wikis/pokemon-omega-ruby-alpha-sapphire/Trace
https://pokeunlock.com/abilities/trace/