Bias Input Current Compensation for Instrumentation Amplifier
The input bias current for these devices is a maximum of 60 pA at 25 degC so, it's unlikely that the 5k6 resistors are there to compensate for this. Worst case is 600 pA at 70 degC so, with that bias current flowing through a 5.6 kohm resistor, the volt drop is 3.4 uV and way below the error produced by the input offset voltage of 10 mV inherent in the "C" type device.

Conclusion: the 5k6 resistors do not contribute to error reduction.

The only other reason I can see that they are present is to allow excessive input voltages that might otherwise damage the chip if the resistors were not present. The data sheet tells us that a maximum current of +/-5 mA is allowed to flow into the input and that would produce a volt-drop across 5k6 of 28 volts.

So, if the inputs (under overload conditions) rose by up to 28 volts either side of the power rails, then the device would still be working within the maximum permissible operating conditions.

My conclusion is that they protect the inputs from over-voltage.