In modern planes there is a filter on the warning computer that doesn´t allow to show certain non vital failures between 80 knots (or 100 depending the plane) and V1. For example an engine fire is going to trigger the alarm, but not an engine EGT (exhaust gas temp) over limit. But the list of warnings not shown, and the ones shown but that must be ignored, are different from plane to plane. That´s why this instructor was doing this test, to make them learn that the 777 had a different list and requirements from the previous one. Mind you that after you´ve been in 2 or 3 different planes it´s easy to react automatically but with the previous plane list.

It´s known that pilots are oversensitive to warnings during take off. A rejected take off is a dangerous maneuver that must be avoided if unnecessary, and is trained in each simulator session. This is more important in wide body airliners like the 747, 777, A340 or A380, as their take off weight is so high that a RTO (rejected take off) will blowout some tires (of the main landing gear) due to the stress or the temperature increase inside the tire created by the brakes. Once the plane is stopped, it has to wait isolated at the taxiway because there are temperature plugs at the tires that may blow out, nobody can get close during that time. If the weight is too high they will even burn as the brakes will be white hot. Look at youtube for a certification RTO of a big plane to see it, or some brake energy absorption test at the laboratory, to see how much energy they have to mannage.

In this case(the simulator story) a after cargo door is one of those warnings that you have to ignore as the plane will flight without problems, then you may turn around and make a landing after dumping some fuel(to decrease the weight).

Hope this helps

Edit: some clarification

Below 80 it is possible to RTO without problems as the speed is still low. Once you reach V1 you will NEVER RTO! as this speed is calculated taking in to account the runway length, and in such a manner that if you RTO at V1, the runway remaining once you have stopped is just a couple dozen of meters. The V1 is so high because that way, when you become airborne, you´ll have a better climb gradient even with an engine fail. All this stuff is calculated with an engine fail, after all it is the most important and probable fail you will encounter (the reverse deployment on take-off is much more severe but thankfully very very rare), and that will affect your climb capability.

Edit 2: added-improved some punctuation.