Mod edit: Rude comment removed.
This is the correct schematic from the Arduino website. I have learned something tonight. And yes as suggested, RSTN is pulled high via a 10k resistor, not low as I initially said. Sorry, I don't know where the other schematic came from.
But doesn't this still mean that I can over-drive it low via a GPIO pin to reset the W5500 chip?
P.S. Why is this not part of the design. What's the point of half of a watchdog? Just the back legs?