Gamma621, that bolded part is not done until the raid is (usually, though sometimes more, depending on raid mechanics) several expansions old, with the resulting growth in character power attendant to expansions in between the initial release of the raid, and the time the "small number of people" attempt it. Vishimtar, for example, was not one-grouped (the definition you are arguing for) for a couple years after DoN, though everyone who heard about it when it first happened was surprised that it even could be done. No one was too surprised when OMM was downed by a single group however, because people had been trying for some time.
A FF14:ARR similarity (though its not a perfect analogy) would result if SE ever decided to give in to the occasional request to allow people to solo dungeons without being level sync'd on down. At level 20 you probably could not solo Copperbell, but at level 40 you likely could.
SDaemon, there are three types of content : solo, group, and raid. Content cannot belong to more than one of those general classifications. Solo content is obvious - you are meant to be able to do it without help from anyone. Group content, apparently, is not so obvious - you are meant to do it with the help of just your group. Raid content, apparently, is causing that lack of obviousness in group content, because people here, prompted by SE's classification, are trying to blur the lines between group content and raid content. And raid content is content intended to be beaten by more than one group. Its that cut and dried.