The idea of core was starting to show its head around 1995. It was not for D&D at first but for other games that were already at their 3rd edition (or more). People at that time were seeing new editions and the proliferation of splat book for what it was, an attempt to grab their wallet. It was thus retroactively applied to any edition of any games.
Core just mean the bare minimum required to play the game as it is.
In D&D it means PHB, DMG and MM (the 1st one). In RIFTS it would mean the 1st book RIFTS. All others are simply splat book that are not really needed to play the game itself. In Role Master, It would be Arm, Claw, Character, Spell and Campaign Laws book (and god we know there were more than that...)
In further discussion around that period, it was commonly agreed that anything beyond the bare minimum to play a game was what should be considered core for any game at all. At least, this is what was going on in my area. The main goal was to provide a "common" ground for rule discussions and to avoid the famous:"I have that book, you don't. So you have to believe me on my word because you do not have it!" This could get very frustrating.
Remember that the internet was in its infancy and a lot of what we consider easy to get today was quite hard. Books were not always ordered in sufficient quantity to answer all demands and second and third orders might let the owner of the store with unsold copies of which he would not get a refund... So store owners were particularly wary about ordering too much stuff. This was the case of our hobby store with 4ed product. It took me quite a lengthy discussion to convince him to give 5ed a chance. He would have stayed with PF at the time simply because he had so many books of 4ed lying in his shop, unsold even at 75% rebate. Even in the first year, it took our Friday Night D&D to jump start the 5ed in our little corner.
So yeah, even if at the time of 1ed everything was considered cored by TSR it is not true any more. And even then, it was a very contested notion simply because not everyone had these books. I personally bought the WSG from a used store, last year...