After both testing & looking at the code, the only potions that can generate in witch huts' cauldron are:

healing - 25%
poison - 25%
swiftness - 15%
slowness - 10%
weakness  - 10%
water breathing - 10%
fire resistance - 5%

It makes most sense for all the witch's throwable potions to be obtainable in their swamp huts, but harming is not present.