Even if Lunge knows Johan exists, who would believe that a small child, recovering from brain surgery, somehow poisoned three people? Criminals lie all the time, and some do either make up, or have, multiple personalities. Anyone faced with this case and not benefitting from seeing all sorts of behind the scenes info as if reading a story would come to the same conclusion as Lunge.
And frankly, even with seeing the situation with everything revealed to readers, I still am not convinced Lunge is wrong. The author better have a real good explanation how Johan poisoned the candy, unless he is going to reveal Tenma really did do it (my guess).