It's already stated in the series that most knights are using mana skin or reinforcement magic. Which is an ability that Asta is mimicking. No reason to assume Asta isn't using anti-mana skin, the anti-magic is
invisible after he activates the form.
Furthermore, Asta made this technique to rival mana skin and one of the goals was to allow him to pass through grand magic regions like the underground volcano. Asta traveled through a grand magic region to the spade base. One he considered particularly
difficult to travel through. So it would be more than likely he was using anti-mana skin at this point, since he just traveled through a tough magic region. This means that the spade captain's poison was strong enough to bypass the anti-mana skin. Giving it potency. This is backed up by being a stage 0 spell.