No, it is not possible to have FD mode but not seeing the FD in the dash 8, so yes they had a command bar showing a descent.
the initial pitch for take off is 15degree, at acceleration flaps are selected to zero, the nose is lowered to the FD GA bar (around 8 degree), then the autopilot is asked for engagement, what will engage the pitch mode, if the nose is not lowered to the GA bar, when engaging the autopilot the FD will synchronise to the current pitch and as said before go to pitch mode.
As it should done, if my FO call AP engagment too soon and not sync with the GA FD bar, I will ask them to go to the GA bar before engagement of AP, then explain why it is important.
You can try as much to understand what happened in their brain, you won t ... read the AAIB report (link at the beginning of the forum). In the world of Dash 8, I m sure it is not the first time pilot take off with wrong setting of FD, it is just probably the first time it went that far before reaction ... But they did appropriate reaction with the ground proximity warning and continued the flight safely.
we can all do mistake, we are all Human ... But we can all learn from mistake of other instead of saying : it won't happen to me, impossible, I can't do this kind of mistake ...