As with every answer; it depends on you

My experience is that when an outsider is coming in, people tend to erase boards of anything they should not see

I would have asked about it and then explained the corrections.

Most likely if you are going to be working in a team setting, they may have wanted to see if you will point out possible mistakes to your coworkers.

There are strange tests in interviews, I remember reading about a shipping company looking for a radio operator. The room was full of applicants and one guy heard morse going over a speaker. It basically said that if you understand this, go to room B. He did that and was hired on the spot.