he did get hit by it. just not the full force, since he used the gates. if he got hit fully by it, he would have diedOrochimaru never survived a tbb. He defended against 4 tailed kyuubi Naruto's tbb, but was never hit by it. He also stated had he been hit by it he would be killed.
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Also jinton destroys on the molecular level and not even Orochimaru can survive that.