There is a difference between electric spacecraft and electrically powered spacecraft.
Elon Musk was saying that we can’t have a solely electrical spacecraft because Newton’s Third Law (for every action, there is an equal and opposite reaction) requires that if we move the spacecraft forward, something has to happen in the opposite direction.
Musk’s electric cars comply with Newton’s Third Law, because they push backwards across the ground to move the car forwards across the ground. But in space, there is no ground to push against. We comply with Newton’s Third Law by ejecting mass. A rocket moves forward by ejecting a small amount of mass at a very high speed.
What Musk referenced, and Newton stated is universally accepted. What you are thinking of is electric ion propulsion, through which electrical energy is used to accelerate and eject a very small amount of mass to extremely high velocity so that, in compliance with Newton’s Third Law, the spacecraft will move forward. Musk is saying we will continue to need to use propellant.