This behavior is by design. The only possible workaround to have a slider on the X Axis or Category Axis would be to manually create the hierarchy first from Insert > Hierarchy, and then select that new Hierarchy on your X Axis or Category Axis
The custom expression for a column with time hierarchy slider is something like below. Since "BinByDateTime" is not a supported function in SQL, manually typing in the same custom expression for an external data table will not work:
BinByDateTime([DATE_JOINED],"Year.Quarter.Month",1)
If a slider is not required, you can also use a custom expression like:
<Year([DATE_JOINED]) NEST Month([DATE_JOINED])>