Actually, even if Meng Hao hadn't concocted the various medicinal pills, the miasma wouldn't have been the least bit harmful to him in any way. The Resurrection Lily had been suppressed, but its nature talent was still within him, which could innately dispel all varieties of poisons.
Meng Hao did not fear poison.
Granted, many years had passed since the Blood Immortal Legacy tournament, which so many Cultivators had witnessed. However, anyone observing could easily make a connection between the two. Thus, he had concocted the medicinal pills to make it seem like they were repelling the poisons. However, their true function was to suppress his own aura, which was was affecting the poison bugs and the miasma.
That of course was something about him that was completely different from the other candidates.