Sorry, but I had to move your post from the "pickups" section.
As for the DM diagram, as you have correctly noted, it makes no sense. However, the bit about the black and white wires being taped together is only one out of three problems I see with this diagram. First, the diagram shows where the bridge pickup's other wires are connected- the red and green wires. But there are no corresponding red or green wires for the neck pickup shown. So, we don't know where the neck pickup is wired.
Also, the caption tells us it's a "2 Hum, 2 P/P" diagram, but only a single push/pull pot is pictured. In baseball, 3 strikes means 'you're out', in this case, it's DiMarzio who's out . . . to lunch, apparently.
The intent of this scheme (insofar as one can tell, given that important wires are omitted) is to put the two coils of the neck HB out of phase with each other. This does not put the neck HB out of phase with the bridge HB, if that was what you were after. I've never found that "intra-pickup" OOP to be a useful sound, but then again, I'm not a hi-gain type, so maybe it's good if you keep your amp turned to "11".
Obviously, someone did a p-poor job of cutting and pasting. Worse, someone else exhibited the same quality of workmanship when double-checking the first guy's work, before posting it to the web or sending it out directly to customers.
The textual labels within the diagram should have noted neck (red) and neck (green) on the switch's two center terminals. Given that, the rest of the diagram reads correctly, including the description below it. Just be aware that since the diagram is bereft of detail, you should assume that the Bridge minus lead (the green wire) will go to ground by default.
The rest of the diagram is indeed correct for your '50s LP setup.
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