In the Primordial Garden, Lex was subject to a lot of pressure because everything around him had the Primordial buff, as he had chosen to recognize it. By the same standards, in Arch-Heaven, he had a major debuff.
Now, in the Origin realm, he was finally able to flex his muscles and really test out how strong he was. As it turned out, he was much stronger than he anticipated, and for two, distinct reasons - his fledgling Dao Body, and his increasing Primordial aura.
Those two aspects added a certain weight to his strength that allowed him to surpass traditional strength boundaries. If Celestials were stronger than Heaven Immortals because they had access to the eight ruling laws, then Lex had access to something one step beyond that - even if the amount of control he had was barely worth even acknowledging.
