Probably because the Monster Manual was the first book released for AD&D (even retaining some D&Disms, such as no AC above 9 IIRC).Sure enough, on the page the player quoted from memory, there was a random parargraph with its own heading buried amongst other non-related topics about animals with tough hides (the paragraph uses the boar as its example) not having to make saves against poison bite attacks. Why wasn't this in the MM entry for the boar?
So I'd assume that when Gary was thinking about boars at the time of writing the MM, he wasn't thinking about poisons. But when he began thinking about poisons at the time of writing the DMG, he thought of boars.
Not a very satisfying answer, but just maybe a correct one.
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