You will clear US customs in YYZ and then enter the US pre-clearance area at YYZ. The area is separated from the rest of the gates at YYZ which is why you cannot get to the KL lounge.
<rest of reply snipped but appreciated>
Ahh! That makes total sense. Thank you. I also looked up the lounge info on KLM's website and it turns out that the lounge doesn't even open until 1200hrs (a full 6 hours after I'll have departed YYZ) so it turns out my question was moot anyway.
But thank you all for weighing in on my question.