Great Old One
Still, power source is more of a descriptive than a mechanically import thing in that edition. It helps classifying things, of course. For instance, Divine classes are usually good at healing even if it's not a Leader. Primal classes get more HP per level than their counterparts of the same role. Arcane classes have elements of battlefield control even when they aren't controllers, and so forth.
I think it really depends as to how you read the rules. I'm not a huge proponent of the RAW in general, but I'm a great proponent of the "fluff", because for me it's the intent that is important, the spirit of the game rather than the exact words. Because, even more than the exact words, the spirit permeates the rules and allows one to understand what type of game it was designed for. For example, I was not surprised that 3e was designed to be "competitive" even if it is never stated in the rules themselves (and for me that was a huge mistake, corrected in every edition since).
Other than that, it's most fluff. Seriously. Just read the Dispel Magic power in the PHB and you'll understand my point.
I don't see why, it even ties in properly with the description of the Detect Magic that I posted, it only works on a conjuration or zone, which is consistent with the first section of Detect Magic. After that, in every edition, Dispel Magic has been fairly specific about its targets, and I must say that the 5e version is not the best one ever either.