Grumpy_Mike:
The datasheet also implies that the external-reset signal is directly connected to pin PC6
No it don't imply anything. the same physical pin can be used as either an external reset OR a GPIO pin 6 on port C. What it actually is can be determined by the fuse setting.
Page 87, the external-reset signal is connected to AIO on PC6.
Page 80, AIO is connected directly to the pin.
If it were otherwise, then why would a pull-up resistor be necessary?