The invincibility of the Xuanhuang Linglong Treasure Pagoda is predicated on it being suspended above one's head; if you cannot hold it aloft, and it gets knocked away by someone, then it's no longer invincible. The Chaos Bell, however, is truly invincible because who in the heavens and earth could possibly knock it down once suspended above the head? Even Pangu's True Form couldn't accomplish such a feat, so who else could?
As a postnatal supreme treasure, its power could never be compared to that of an innate supreme treasure, not to mention innate spiritual treasures, such as Zhun Ti's Seven Treasures Wonderful Tree or Amitabha Buddha's Twelve-Rank Lotus Platform, whose powers are both incredibly formidable. Even if not stronger than the Xuanhuang Linglong Treasure Pagoda, they are at least on par with it. In comparison with the Chaos Bell or the Zhu Xian Four Swords, the disparity is just too vast.