In his Lotus form, Lex could technically completely absorb the energy and transform it into something non-harmful. That held equally true for the Chaos energy wreaking havoc in his body. Even if he ignored that, Lex could just strengthen his body's defenses, for he had trained in the Chaos Sea within the Midnight realm, so he had excellent resistance to Chaos energy.
He did none of that. Though, from a certain point of view, it seemed like his body was being destroyed and burned from the inside out, the truth was, this was exactly how tribulations helped the target grow.
His cultivation technique was unique in that it allowed him to grow as and how he wished, which meant that in theory, there should have been no bloating or waste in his body. That, however, was only true while he was an Earth Immortal.
