It seems that Cooper assumed some basic common sense while the NRA dumbs it down for those without common sense.
There is some validity to that - NRA sought to develop easy to understand, universally applicable rules that are simple for both novice and expert to implement.
As noted, the major issue with the "Cooper" rules is the "all guns are loaded" assumption. For the expert or journeyman, that might be a useful caution - for someone inexperienced with firearms, it's not instructive.
In practice, NRA 1 and 2 inherently assume a loaded firearm and direct "Always" appropriate action.
3 simply directs that operators unload firearms not under their direct or indirect, immediate control - which cause the majority of unintentional shootings.