These are two different questions.
One is
“Do you know who the tallest person in class is?”
vs.
“Who is the tallest person in class?”
In the original example, the person asking the question already knows the answer, so it doesn’t make sense to ask “who” they are but instead “if they are known” to the listener. This is evidenced by Aiko responding that the person Tanaka listed isn’t in fact the tallest in class.
This is why I think the second question is actually the correct rewrite here, even if it does sound slightly odd.
Thank you for the reminder!