What is the lowest resistivity that can be measured with the 8009 resistivity test chamber and 6517B electrometer?

It depends on the voltage source and ammeter, not so much the test fixture.
The Model 6517B voltage source is capable of sourcing 10mV to 1000V. And up to 10mA.
So this means if it sources 1V and has a 10mA compliance then it could read 100 ohms.
Surface resistivity of 53.4 time the 100 ohm would be 5.34k ohms/sq.
Volume Resistivity of 22.9 times the 100 ohms would be 2.29k ohms-cm.
So in general the 8009 should only be used for resistivities in the 1e3 range and higher.