To be fair, none of the other books had been published yet. Again, we're talking about the specific term "Core Rulebooks". That is a term that didn't exist as it's meant now, before the 3e books were published. LIke I said, the Core Rules CD included the Tome of Magic and the Arms and Equipment Guide.
The idea was that if you were playing D&D, anything that wasn't specifically tied to a specific setting - as delineated by the trade dress - was core. Here's the opening lines from the 2e Complete Fighter's Handbook:
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That doesn't really sound like a book that isn't part of the core set of rules. These aren't new rules - these are the rules they would have given you but ran out of space for in the PHB. Also, one should note the coding for these books - PHBR. As in Player's HandBook Reference. I've also seen it referred to as Player's HandBook Rules. Either way, that's not something that isn't part of the Player's Handbook.