ID refers to drain current, not diode current. It's normal to characterize Vth at a very small fraction of the rated maximum drain current. This is so that the designer knows they have to keep Vgs below that level to avoid burning the FET due to incomplete turn off.
For example if your FET operates from 400VDC and the gate driver only pulls down Vgs to 1.8V, then a few milliamps of current at 400V may cause more heating than the losses when the gate is driven high.