With respect to the question posed by Cthulhudrew, I'm thinking "no" (there was no integration intended) for a few reasons.
First, he says the Master's Set map is already in existence, and that map makes no mention of any of the wilderlands nations, and none of the coastlines are compatible with the wilderlands.
Second, by Sep 1983 when this was published, Judges Guild had lost its license to produce AD&D materials, so I doubt that anyone at TSR would be promoting or using any JG material.
Third, I believe the JG license was for AD&D (?), and by 1983 TSR was being very careful to keep D&D and AD&D separate.
That said... Mentzer says in this article that the X1 example map is now an "official" game world for the BD&D line. Previously it was only a sample wilderness. I was rereading Let's Read The Known World/Mystara
, and modules published in 1981 by Moldvay pretty much avoided explicit placement in the Known World. He even went so far as to scrub out KW references in the revision of B3. By 1982, X4/X5 were expanding the setting, but Mentzer was surely already at work on BECMI when these were being written, and making the setting official was probably already planned. Still, in 1981 there was no official setting, just a sample wilderness. I know when I started with Expert as a teenager, that is how I took it, and I started creating my own worlds using it as a pattern, rather than thinking of it as official.
If there was no official gameworld for BD&D, and JG was publishing officially licensed AD&D product until 1982, then the Wilderlands might have been perceived as a "default official setting" for D&D. Lots of people were using JG modules in their settings, and presumably using TSR modules in the Wilderlands.
In that sense, if a lot of people had played existing campaigns in the Wilderlands, then they would be familiar with the Overlord as a super-high-level, nigh untouchable NPC. There was no way 14th level characters could hope to challenge him. The Companion and Master set promised to allow levels of power that would exceed the Overlord himself. Mentzer also hints at the Immortals Set (which he suggests here is the official revision of Gods Demigods and Heroes) that definitely trumps any mortal overlord.
Also, any existing campaigns in the Wilderlands using D&D rules (OD&D, Holmes, B/X) would not be expected to switch to the known world. Mentzer is clearly addressing existing experienced players here, who already have a campaign world, and is not assuming that anyone is using the KW as a setting. If they are already playing in the Wilderlands then the Overlord does reign supreme and does stand to be deposed. That does not imply a connection between the KW and the Wilderlands, so much as it implies the popularity of the Wilderlands as a setting in 1983.
So so summarize I think he may have been just name-dropping. If the Overlord was commonly perceived in the community as the pinnacle of power (in any gameworld), Mentzer is saying that is no longer true.