Consequences of his genius
According to Lex's understanding of how tenets worked, the more he used it to influence a law, the more it would be drained. The greater his own understanding of that particular law, the less the strain would be, whereas conversely the less he understood a certain law, the more strain he would have to endure.
Lex also believed that there had to be some way to strengthen his tenet, though he did not yet understand what it was. The stronger his tenet was, the longer he could influence laws, and to a greater extent.
Based solely on those things, Lex assumed that the black pillar was actually some obscure law that Lex had never encountered before, or did not understand. The other immortals were, somehow, strengthening or boosting it to the point where there was no other law around him, and no escape for him.