How to fill down in Numbers without dragging?
Found a way. 


Select the column 
Deselect the header row (Cmd click on the header
row cell)
Table Menu > Autofill Cells > Autofill Down


This will ​not​ infer a pattern. (ex 1,2,3 it will just copy down 1,1,1)

If you’re expect it to interpolate, you have to click-drag.

It will fill down formulas and they will be correct.