All according to plan
While Lex's fire tribulation occurred, he could not control any spiritual energy at all, since even his meridians were burning. In fact, since his muscles were burning as well, he should not have been able to exert much strength either.
In reality, he was already physically weaker than he was when he fought against the lightning bolt. But Lex had the confidence to dominate against anyone in the immortal realm, at least when it came to physical abilities. Once matters of laws got involved, he was not arrogant enough to think that he could suddenly surpass everyone.
After all, even if he had been playing with laws for a few years, most immortals had been learning about laws for at least a few decades or centuries.