[No, the problem isn't that Mikami is overestimating Light, it's that he's underestimating him. If it turned out that this was a setup, he knew he could twist it to make following this man a foolhardy but ultimately well-intentioned move. The prosecutor assumed Light had made a mistake be following if he still suspected him, but he was really just unable to see far enough to know how easily Light could talk himself out of situations. A tongue woven from silk had advantages.

However, things really did begin to unravel as the man spoke. Rather than an enemy, he sounded like a less than sane man who had somehow managed to gain knowledge about the Death Note. He knew the Shinigami's name, but he also claimed that Light had sent him. Either there was a miscommunication or this man's inability to think straight was a bigger problem than he thought.]

While I think it's impossible to have confused me with someone else, I haven't met you before today. I've had neither means or reasons to contact you.

[His mind wasn't going to assume the man was from the future. A Note that killed people was already stretching his beliefs.]