How Are the Ackermans So Strong?

Thanks to an "awakening power," which gives them physical strength far beyond humans' normal capabilities, the Ackermans are able to channel the fighting power and experience of their ancestors, essentially utilizing the strength of Titans without becoming one themselves.

Why do Ackermans have powers?

They were the accidental result of the Eldian Empire's experiments with the Subjects of Ymir and Titan science. They were designed to protect Eldia's king, and acted as the monarchy's right hand, entrusted with the survival of their people.

Why are the Ackermans immune?

As mentioned above, the Ackermans are immune to the power of the Titans -- their memories can't be erased by the power of the Founding Titan. The reason for it is that their bloodline has been modified, as they are no longer considered as Eldians who can turn into, and/or affected by the power of the Titans.