Am I understanding this correctly? When the attach() method is called in the following code snip, the returned value is a pointer to this Servo instance's attach overridden function?
No, it's returning the result of the call to the attach() method that takes three arguments.
'this' is the pointer to the servo object. 'this->attach()' is a call to one of the attach() methods of the object. The particular attach() method is chosen by the number and type of arguments.
This function makes myServo.attach(pin); equivalent to myServo.attach(pin, 544, 2400);