The posted video is/was about something. Is or Was?
Solution 1:
Since your first question to your teacher mentions "today's" video, it sounds like the video is still posted and you are wondering whether it is a mistake and, most important, whether the material you are supposed to be viewing is actually available.
So we're still in the present. I would therefore suggest "The video posted is..." The other choice is not grammatically wrong, but it's clearer to use "is".