I build hospitals for a living. The systems are not sealed in the manner you are referencing. Negative Pressure means exactly what I explained above.
You do not understand negative and positive pressure. Pushing hot air out and getting cool air in has little to do with positive or negative pressure. Hence the reason for air balancing. You can be moving 300,000 CFMs thru a room with no pressure change, provided you have the ducting and equipment sized correctly. Positive Pressure means the air pressure inside of a controlled space is higher than that outside. Negative pressure means the air pressure outside of the controlled environment is at a high pressure than inside the door. Think of an extreme case of positive pressure as a tire under inflation. That is positive pressure.