Voltage Referencing Question
The source of the bottom FET is the negative reference for both Vgs and Vds.  This does not imply that Vgs is equal to Vds.

You can consider the bottom terminal a "Zero Volts/Ground" if you wish.  (Actually, you can consider any point in a circuit as "Zero Volts", but in this case using any point other than the bottom terminal as "Zero Volts" will only cause confusion.)