Warning: SPOILERS for Superman and Lois season 1, episode 7, "Man of Steel".
The Stranger, a new villain created for Superman and Lois, was teased as being an alternate version of Lex Luthor, which is fitting considering a particular Clark Kent development. While he was ultimately revealed to be someone else in the episode "Man of Steel," he does share one trait with Lex Luthor; a complete inability to see that Clark Kent is Superman. However, while Lex made this mistake out of hubris, the Stranger seems to have made the error simply because he never stopped to consider the possibility of Superman having a secret identity.
Few details were made clear about the background of the Stranger as Superman and Lois built up to the revelation that he was an alternate universe version of John Henry Irons, whom comic readers know as the superhero Steel and an ally of Superman. One fact that did come out was that, in his world, John Henry Irons was married to Lois Lane, which led him to help the Earth-Prime Lois Lane with her investigation into corrupt billionaire Morgan Edge. His offer of aid made Lois suspicious, however, and Clark Kent broke into John's mobile home to see if he could learn more about the man while Lois met with him in public. When John's Artificial Intelligence addressed him as "Captain Luthor," Clark knew that something was up.
The artificial intelligence reported Clark's burglary when John Henry Irons returned home, showing an image it had captured of Clark. He recognized the intruder as Lois' husband on this Earth, but dismissed him as "not important" when the AI asked if Clark was a threat. Ironically, John had met with Lois trying to arrange a meeting with Superman, having no idea how close his sworn enemy was the whole time he had been stalking Lois and working up the courage to approach her. Funnily enough, John's mistake regarding Superman's identity echoes Lex's from the comics.
While John Henry Irons is not a Luthor, his inability to see the obvious in regards to Clark Kent is a Luthorian flaw and a callback to the Superman comics. Many stories have played up the fact that Lex Luthor, being the narcissistic egomaniac that he is, cannot fathom the idea that anyone could have the power of Superman and be content with living a simple life. One of the most famous examples of this came in Superman Vol. 2 #2 in 1987, where Lex commissioned a team of detectives and a supercomputer to figure out Superman's secret identity, only to refuse to accept the results when they all concluded Clark Kent was Superman, even providing him with a stolen scrapbook made by Martha Kent, filled with newspaper stories involving her son's heroic activities.
The Lex Luthor of the Arrowverse showed a similar level of stubbornness in regards to his arch-enemy. Despite determining that Kara Danvers was Supergirl and being more than capable of connecting her to Clark Kent, Luthor refused to believe that Superman would ever pretend to be someone so ordinary. Even after acquiring the reality-altering Book of Destiny and going on a multiversal killing spree where he murdered multiple versions of Clark Kent, Luthor could not believe that his Clark Kent, the Clark Kent of Superman and Lois, was the Man of Steel.