Take this into consideration:

Some people lie, or give partial truths, even in low level content, so that someone doesn't drop from the group should they admit to not knowing, or are familiar with the dungeon and have to wait 30+ minutes on the DF again.

I tend to say that I've watched a video, because I usually have. But that doesn't mean I'm intimately familiar with fighting the boss or progressing through the dungeon itself. All the video does is give you an idea what to expect and how to deal with it. However, I've seen some people drop group because things didn't go 100% flawlessly (and they were the one responsible, not the people that they are verbally abusing).