What did David's words mean at the end of Project Almanac?

It's left purposely vague, so it's up to the viewer to decide what the line meant.

Even if they don't remember any of what happened, one of the video cameras has the entire sequence of events on record.

That's what I got out of it: presumably David and Chris watched through the tape on the camera to see what happened. They saw basically everything we saw in the movie. To me, that last line meant that they were at least going to tell the rest of the group about what happened. From there, maybe they'd release the footage. Maybe as a movie...?

An in-universe explanation can be that he said that because of the stuff that they were going to do, like build a frickin' time machine, mess up the timeline and everything that they did in the previous timeline, as it all recorded on the camera of the previous timeline.

An out-universe explanation is as @Valorum told in the comments, that it was bad writing.