The premise of cryonics is that who you are, your memories, personality and sense of self, is encoded in the physical structure of your brain: the pattern of connections between neurons and the molecular state of those connections.
This is the mainstream view in neuroscience. There is no evidence that identity depends on continuous brain activity; people recover their full selves after deep hypothermia, cardiac arrest and anesthesia, during which electrical activity stops entirely.
If identity is structural, then the question for cryonics becomes whether preservation keeps that structure intact well enough to recover. That is an engineering and quality question, not a question of whether the self is somehow lost the instant the heart stops.