My pleasure! Was there a specific question asking to explain withdrawals? Some secondaries have it, but several I have seen so far have not. If they don't specifically ask, there is no reason to go there, especially because it's such a small thing. Of course, if they do ask, then explain. There is no reason to try to hide it and potentially raise questions by doing that.
Again, not ideal if they did ask and you didn't tell. After the fact, though, it's too small to make a difference, and I think the risk of negative fallout by highlighting carelessness trumps whatever benefit you might receive by correcting the record. The class is clearly marked as a TA, correct? If so, withdrawing from that is not exactly like withdrawing from orgo!
What they are really looking for with the question is an explanation for withdrawing from a whole bunch of classes, like a family or medical emergency. Telling them you withdrew from a single class because you were feeling overwhelmed, or, in this case, because you were mistakenly enrolled in something you weren't actually participating in, just isn't going to matter to an adcom, and it's not really the point of the question.
The potential negative here is totally not the withdrawal. It's the carelessness by forgetting to mention it when asked. This is why I wouldn't open this can of worms.
