Not saying I'm correct, but don't misuse Occam's razor in this.
I'm not using it technically, but I'm using it correctly. What the Wikipedia quote you presented leaves out in its refutation of the "common misinterpretation" is the idea of "all else being equal."
As in the simple statement
here.
All else being equal between your guess and his, you are "multiplying entities" by making more assumptions.
I think that they wanted a druid to have armor they could wear without violating their oath, so they chose a suit of full plate and made it dragonhide as an example of armor intended for druids, forgetting that that druids aren't proficient in full plate to begin with.
They didn't forget. (Seriously, you think that the designers of the game remembered the religious prohibition for druids, but forgot their proficiencies?)
They simply didn't need to specify that proficiency was necessary, because there are two separate rules governing the wearing of plate armor for druids: religious oaths and proficiency. Religious oaths have to do with metal, so they're simply saying that the religious oaths won't be a problem. Proficiency is separate and there's no reason to call out that the rules continues to apply, any more than it's necessary to call out that ACP continue to apply.
I mean, seriously, if I wrote, "Your rogue can wear full plate if he really wants to," do I really need to say, "Of course, it's probably not a good idea, even if they're proficient in it"? And if I don't say that, would you really assume that I forgot about proficiencies and ACP?