She was told everything anyone probably ever knew (at least for the past few thousand years) from first hand encounters by Jon.
The way the whole scene was set up didn't make any fucking sense. She came with her three dragons, and it was clear that just one already could plow through hundreds of wights in a heartbeat. She could have cleaned the place within a couple of minutes, instead of fumbling around trying to get 10 people on a dragon's back while they're still fighting with wights.
Make the NK kill one immediately, making her realize that they need to bail. Make her go with 2, leaving one at Dragonstone just in case, so with one dead they really need to bail. Make her try to burn the whole place down and the NK pull some magic to stop them. It could have been handled in ten different better ways. The way they went was really dumb, all for the sake of a few minutes of shiny CGI and "tension".