Yes, it can be driven directly with no input circuitry as long as the current through the LED doesn't exceed the maximum rated current.
The input circuit is there to limit the amount of driving current through the LED to a maximum of 10mA across the whole input voltage range of 3 to 30V. This allows users to interface to a wide variety of controllers and signals without having to worry about burning out the LED from over-current.
The circuit also generates a short 15mA spike at turn-on to help turn on the SSR faster by driving the LED harder for approximately 500us. The current then drops to 10mA to keep the SSR turned on.
There is also reverse polarity protection and ESD protection.