It appears that the NPN pass transistor is also used as a switch. The only path for current is thru the transistor, and thru the IC.
I can't find the datasheet for this IC on the internet. Chipdocs.com has it, but they charge a price to register to view their datasheets.
If the IC is configured properly, it appears that there can be zero volts output on this variable power supply. The current thru the transistor can easily be turned off by turning off the voltage to the base. When there are zero volts at the base, there will be no current thru the transistor, and no voltage from it, the transistor is turned off, and it is like an open switch.
The only other output path is thru pins 1, 2 and 10 on the IC, and it depends on how the IC is configured. It appears that 1, 2, and 10 are some type of sensing feedback paths. It also looks like pin 6 is the voltage output, and pin 6 is connected to the zener, which goes to the base of the transistor. The zener probably drops the voltage output below the normal minimum output down to zero volts, which would make the circuit capable of a zero volt output.
But without the datasheet for the CA723CT, this cannot be verified. You could probably email the author for more info. Since there is a link to his e-mail address, he probably wouldn't mind.