Okay, PaintCAD
® again
Black is ground.
Pink is the zener
and a compensation capacitor
Red/magenta is the bandgap NPNs and emitter resistors.
Orange is PNP buffers.
Yellow is the active load.
Green is load for the orange buffers and a first level shift.
Cyan is more level shift and active load (outside the view).
Blue is the VREF output darlington.
Violet is current limiting.
Brown is the feedback divider.
VREF voltage is Brokaw output voltage (red collector) + orange Vbe + green Vbe + cyan resistor drop + cyan diode - 2x blue Vbe.
Approximately it's red collector + 1x Vbe + cyan resistor drop, so red collector is a bit less than one Vbe below VREF. And the zener clamps this voltage.
edit
I'm not saying that this is not a bandgap reference, pretty surely the BG circuit wouldn't be here if it weren't regulating the output voltage.
But the zener is reverse biased close to breakdown and appears to really be a zener and not just capacitance.
It may be handling edge cases like startup, load transients, VCC transients, although I can't imagine what exactly it does