Liu Er did not feel strange about it, as the three were senior disciples of the Humanity, Enlightenment and Severity Tribes respectively. It would be strange if they did not have Magic Weapons for protection. For example, though the Guang Chengz's Smashing Seal was only a Postcelestial Spiritual Treasure, its force was no less than that of a Primordial Spiritual Treasure. Given that it could be refined with the Cauldron of Heaven and Earth, it would equal a high ranking Primordial Spiritual Treasure exactly.
Moreover, Liu Er had not retaliated with all his strength. Otherwise, Xuandu may not have been able to resist the power from his punch even with cultivation at Primordial Unity Golden Immortality and the Floating Flame Flag. Like Minghe, Liu er paid great attention to refining his body, his strength was the strongest it could be.