In fact, he actually could do it!
Although the various Physiques were indeed inferior to weapons of the same level and in order to break a Dharma treasure, the Dharma Treasure had to be of a lower level. After all, flesh and blood were still inferior to solid artifacts.
It was just like how a human's nails and bones could block the blade, but flesh could not.
Although his body now possessed spirituality, it was still inferior to spiritual materials that Dharma Treasures were made of.
Therefore, although the Spirit Body had the word 'Spirit' on it, it was still inferior to real Numinous artifacts or even Numinous treasures.
His inferior-grade Spirit Body could only match with inferior-grade Dharma treasures without the help of any means or Dharma treasures. However it was still rather powerful.