For example, how would I represent the following rule?

"Combine ingredients A and B if A is not vinegar and B is not baking
soda."

After reading through the docs, the closest thing I could come up with
is:

combine_ingredients
    foreach
        ingredients($A,$B)
        notany
            is($A,vinegar)
            is($B,baking_soda)
    assert
        combine($A,$B)

However, I'm not clear how multiple statements under the notany
keyword are interpreted. I'm assuming they're ORed, which means that
that representation would not fire for the statement ingredients
(vinegar,vinegar), even though it should.

Multiple clauses under a notany are AND-ed (like multiple clauses everywhere
else).

  notany
    clause_A
    clause_B

means that there is "not any" proof for "clause_A and clause_B".

So the rule that you have should do what you want.  (And, as stated, would
combine baking soda with vinegar, just not the other way around).

Two things that you might consider with this example.

First, if the ingredients don't conceptually come in pairs you could use one
list of single ingredients twice.  Second, you could use a check clause with
Python's != operator:

combine_ingredients
  foreach
    ingredient($A)
    check $A != 'vinegar'
    ingredient($B)
    check $B != 'baking_soda'
  assert
    combine($A, $B)