Look at the pinout for a '328,
When the DIP part is installed backwards, VCC/GND from pins 7/8 and up at Aref/GND at pins 21/22,
and Aref/Gnd end up at pins 7/8.
Avcc ends at PD4, and PD4 ends with Avcc.
I think that's what save the part from being damaged. Vcc and Avcc are not shorted out, Gnd is still at Gnd.
The bootloader/sketch are not started because the oscillator is not hooked up.
I've done this myself also (and have now marked my boards as well) and have not seen any damage occur - the chip does not get warm even, which is usually a good sign of excess current being drawn.