I wondered a bit about that too, so I looked it up, here is one explanation:
"Sidestepping decades of precedent, Musk says that Starship will have no separation mechanism at all. Instead, at some point during the design or testing process, Musk decided that a separation mechanism was entirely superfluous and that the same effect could be more or less replicated by using existing systems on Super Heavy. By using the booster’s gimballing Raptor engines to impart a small but significant rotation on the rocket moments before separation, Super Heavy could effectively flick Starship away from it – a bit like how SpaceX currently deploys Starlink satellites from Falcon by spinning the upper stage end over end and letting the spacecraft just float away thanks to centripetal forces.
Because Starship is something like five times heavier than Super Heavy at stage separation, the ship would effectively float away from the booster in a straight and stable line, use cold gas thrusters to settle its propellant, and ignite its six Raptor engines to head to orbit. In return for the slightly unorthodox deployment profile, if this new approach works, SpaceX can entirely preclude the development of a pusher/spring system capable of pushing a ~1300 ton Starship away from Super Heavy. That approach is possible on Starship in large part because the ship’s six Raptor engines are completely tucked away inside a skirt, meaning that there is zero chance of nozzles being damaged by impacting the booster interstage."