When Lex says these words to Clark, he keeps glancing at the security camera. He knows it's there, and his back is to the camera so that no one can read his lips - but even with the chaos all around, with the kind of technology to which money like Lionel's has access, it's potentially audible.
Does Clark know the camera is there? We can't be sure if he does, but Lex's glances could certainly cue Clark to the fact that they're there. Could Lex be drawing Clark's attention to the existance of the cameras in an attempt to make Clark believe that he was avoiding being seen, heard by hidden eyes and ears?
But what if Lex's words weren't to assure Clark while avoiding being overheard? What if he mentioned Clark's secret in a vain attempt to get Lionel to overhear? If Lionel knew that Lex knew Clark's secret, something Lionel has been chasing probably since the Kents found his son in the cornfield, he'd want the information Lex had, *need* the information for his own obsession. Then, Lex would be worth something to Lionel, if only as a database. No longer just a threat to Lionel's freedom by keeping the secret of the hired murder of his parents, Lex would be a commodity to Lionel, wanted, needed, valued... and safe.
At this point, Lex may or may not have known Lionel's plans for him, but if he had a secret Lionel would want to keep safe, Lex had a life insurance policy - either Lionel would be forced to put to a stop to whatever dastardly plans he had for Lex, or he would remove Lex from the asylum so that this second secret could be kept from prying ears and available for Lionel's perusal and exploitation.