If advertising costs go down because of AI, then advertising prices will go down too. That money then goes somewhere else entirely, but anyhow gets spent.

If there seems to be more money around for various causes than before, that looks to the market as if money supply has increased. Which definitely will cause inflation.

Business seems to be getting more profitable because of that applied math, so investing in it seems smart. True deflation would see value of money to shoot up so quickly, that you are better of if you just stuff cash under your mattress.