Having no experience on this type or indeed any FBW aircraft I have no idea of the relative "feel" of the controls A/P engaged or disengaged.
Given the situation they found themselves in , I don't find it too surprising that they could have missed this, assuming the control feel is similar.
On something like a B737 it would be fairly self evident , as the controls (even in CWS) do not feel the same, and anyhow the aircraft would have wandered off on a tangent, but with FBW, gust alleviation, and all these other boxes of tricks between you & the hardware ? perhaps someone experienced on type can tell us.