Its a great library i have used along with Events Library , the Input Shield and LCD to create a Prototype Controller for my Rover Project, allowing me to control the motors and also receive the sensor data.
Can the same type of communication be done between Processing & Arduino?
The Arduino hasn't a clue where the data on the RX/TX pins came from. It could have come from another Arduino, from a PC, from a GPS, from a XBee, or from any of dozens of other sources.
That library simply defines a protocol that both ends of the communication link can understand.
Looking at the source code, you should be able to figure out how to make Processing send data the same way.