Forgot about the first two lines you mentioned my bad. Most of the grass types are also poison types in gen 1 though. 9 of the 14 grass types are also poison and that's why they there were so many poison types in gen 1.
I don't think it's fair to say that the Grass/Poison types are the reason there were so many Poison-types in Gen 1. 9/33 is only ~27% of them, which means ~73% of the Poison types in Gen 1 were not part Grass.
Edit: In all fairness though, all other Poison combos do have less than 9, so Poison/Grass is the most common Poison-type combo in Gen 1. There are 10 pure Poison, 5 Bug/Poison, 3 Ghost/Poison, 2 Ground/Poison, 2 Flying/Poison, and 2 Water/Poison.
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