An anti-slapp law provides a short-circuit motion to dismiss. But the legal merits are still evaluated–just much earlier. An anti-slapp motion basically says: "Even if everything the plaintiff said were true, it's not legally actionable so end the case now".
Qualified Immunity is insane because it doesn't require a legal evaluation of the case. Qualified Immunity (practically) says: "because this exact fact pattern hasn't been tested before, the officer couldn't have known it was specifically wrong. Since the officer couldn't have known it was specifically wrong, we don't need to go any further".
That means, a case dismissed because of QI doesn't even end up demonstrating that the fact pattern was legally wrong!