This is the 13264th most frequent Korean word.
들도
"들도" means "even they" or "also they" depending on context.
Here, '들도' adds the meaning of 'also' to 'students', indicating inclusion of students within the group performing the action of studying diligently.
학생들도 열심히 공부합니다.
The students also study diligently.
In this sentence, '들도' refers to 'experts' and conveys that they too share the state of confusion described.
이 문제는 정답을 모르는 사람들이 대부분이지만 전문가들도 헷갈립니다.
Most people don't know the answer to this question, but even experts are confused.
The word '들도' here emphasizes the group of 'those who knew the song', indicating that even they danced, expanding the scope of individuals dancing.