Ok, i see the confusion.
In your airline you select the SID alt minus 100ft then press GA, HDG and alt sel.
in flybe, the alt is set to zero, clearance is taken then the PF should set the SID altitude, then press GA, hdg and alt set then calling it, PM should check it.
what they did was, they took clearance, then went straight to press GA, HDG, Alt sel then set the SID altitude, so when alt sel with the altitude selector at 0 was pressed, plane went in alt locked mode.
Then now have a FMA saying HDG and alt locked with the altitude selector set at the SID. So you won't have any alt warning and altitude amber ...
Ofc, the FD was showing a horizontal bat at zero pithc in the ground and not +8 degree with GA mode, thet could have spotted it on the taxi checklist and they could have spotted it on the line up check list (check list ask to call FMA mode), was no alt sel arm white at all so it could have been spotted ... then when passing acceleration altitude, they could have seen the FD showing a deep descent and could have spot it there ... We end up about brain is good to see what is used to see and disregard what doesn't fit in his idea of reality.
How you explain the Emirates 777 with 3 crew trying to do a go around with idle ? They were used to have go around power set by autothrottle ... Expect in that case autothrottle was off due to the plane touching rwy...
Or how explain the airbus A320 in "mont Saint Odile" in France, instead of FPA 3degree they selected VS 3000ft/MN, they then we're focused of why the plane was speeding up ? I do remenrem one of the crew convincing himself saying, oh we have little bit of tail wing and plane really light today .... Brain see what he want to see ...