Doesn't say anything about Planescape though. Doesn't use the Planescape names for the planes (Nirvana instead of Mechanus, for example). Also, as a MC, still a core book. Unless you're going to claim that a particular date is suddenly the magical cut-off for canon.
Well, to be honest, the idea of core is a bit fuzzier when you're talking about AD&D. In AD&D, everything was assumed to be core. At least, that was the intent. It wasn't until 3e that you get a very sharp demarcation between core and not-core.
But, in any case, we're still talking three years of 2e before you get any Planescape material at all.
In fact, 3.5e is the outlier here. 1e obviously had no Planescape, although it certainly had the Great Wheel cosmology at least outlined. 2e had no planar cosmology at all on release and it was years before you got any. You couldn't have the Great Wheel when you don't have any good or evil outsiders to speak of.

Granted, once they decided that Planescape would be the meta-setting that unified all the settings under one umbrella, Planescape went great guns and started showing up everywhere.
Forward to 3e. All that Planescape stuff? Gone out of core. It's barely even mentioned. Great Wheel is there, barely, although there's nothing really mentioned about it specifically in the Core 3 books. Planar elements are pretty much gone and there certainly isn't any mention of the Blood war or anything resembling Planescape lore. Heck, the aforementioned Rod of Seven Parts isn't even in the 3e DMG. (I just checked).
It's not until 3.5 that Planescape gets to appear in core books. And, then, it gets to infiltrate virtually every planar supplement from that point forward. It's all Planescape ALL the time. Even in things like Book of Exalted Deeds have Planescape elements showing up. Every Dungeon adventure dealing with the Planes gets the Planescape seal of approval. As Shemeska said, if you wanted to publish anything for D&D that dealt with the planes, you had to go Planescape.
Then 4e rolls along and resets the baseline again. Planescape lore gets ejected. Although, this time, it gets replaced by something else. I'm not sure if that was a good thing or not. But, at least it was something different instead of the same old death grip that Planescape lore has on everything else.
But, the idea that Planescape is the baseline for all things planar in D&D is only true if you presume that latter era 2e and 3.5 D&D are the only D&D's out there. Because it sure isn't true for the rest of the game.