XU RU could tell that there was something obstructing his vision and fooling his senses. Among the members of team Yunyue, he couldn't think of anyone who could have illusion skills. But with the process of elimination, it's easy to guess who it was. The fox pet of Noctis.
Which meant he was in the right place. Because either the fox was protecting this area until Noctis arrived, or Noctis was already there and this illusion was so no one here would be able to get in the way of Noctis putting the relic in its designated place.
Whichever it was, it didn't matter. All Xu Ru had to do was to destroy this illusion.
Ilusions were called illusions because they were not real. So, what he's seeing right now had no physical form. Meaning, if there's a wide-area attack that could bombard this place right now, that fox would definitely be hit. Not that he had an AoE skill for that. He's an Assassin, AoE attack was not his thing. But he had an item that could give the same effect.