70% infection Suggests a LOT of pre-existing immunity.
Moreover, even if 100% antibody + rates does not rule out a lot of pre-existing immunity. If I am naturally resistant to coronavirus (e.g., COVID patient sneezes in my face and I get a mild fever 10 days later), I'll probably develop antibodies from an exposure and otherwise be fine, I'd call that pretty damn good pre-existing immunity. According to your logic, that would be "no [pre-]existing immunity at all."
Moreover, even if 100% antibody + rates does not rule out a lot of pre-existing immunity. If I am naturally resistant to coronavirus (e.g., COVID patient sneezes in my face and I get a mild fever 10 days later), I'll probably develop antibodies from an exposure and otherwise be fine, I'd call that pretty damn good pre-existing immunity. According to your logic, that would be "no [pre-]existing immunity at all."