Learn Finnish: Oletko varma siitä huolimatta, että argumentti on kyseenalainen? - Are you certain despite the fact that the argument is questionable?

The rule about evaluative adjectives taking the partitive (like on tärkeää, että...) applies when the adjective describes an entire että clause acting as the subject. In this sentence, kyseenalainen isn't describing the että clause; it simply describes the noun argumentti inside the clause. Since argumentti is a singular nominative subject, its adjective must agree with it in the nominative case.

Why do we say siitä huolimatta, että instead of just huolimatta, että?

The word huolimatta (despite) requires the word attached to it to be in the elative case (-sta/-stä). Because you cannot attach a case ending to an entire että clause, Finnish uses the pronoun se in the elative case (siitä) as a placeholder. The että clause then spells out exactly what that siitä refers to.

The adjective varma also usually takes the elative case (like olen varma asiasta). Is the siitä in this sentence attached to varma or to huolimatta?