Correct. But once iginition happens, you no longer have a vacuum in a chamber that resembles the “endless, boundless absolute vacuum of space”, so the leaky bucket analogy doesn’t hold.
I’ve just stated that prior to ignition, the pressure in the thrust chamber is greater than zero. Once ignition occurs, the pressure and temperature rise so that the mass flow in equals the mass flow out (flow equlibrium). So, the only difference before and after ignition is the temperature and pressure in the chamber, and in both cases, the pressure is greater than zero.