If Lex had to guess, based clearly on the remarks, it basically meant that the Nether was somehow responsible for providing energy to the universe. Considering the fact that the notification mentioned absorbing, recycling and rebuilding, he guessed that the Nether somehow absorbed the wasted energy from the universe, or maybe outside the universe, and adapted it so that it could be used within.
Such a simple explanation made sense to him, and considering the complexity and scale of the universe it was not unbelievable that the universe had its own dedicated power source. The thing about the Heaven and Mortal realms was unclear, but it could also potentially be explained if he treated Arch-Heaven as the Heaven realm, and the rest of the universe as the Mortal realm, although that was also contrary to his understanding of the universe.
Also, as far as he knew, the Nether was a part of Arch-Heaven, so there were some serious flaws with such an explanation.
