Open Drain while transmitting data with port direction output
When a chip drives a line, it forces the line to have a specific voltage by making a direct connection between the output pin and that voltage (in your case, either 5 V or ground).

When a pin is in HiZ mode, the pin is not connected to any voltage, and the line's voltage is determined by some other chip driving the line, or by the pull-up resistor.

The schematic below shows how the chip might actually be implemented. When the switch is closed, the chip drives the line low, and when the switch is open, the pin is in HiZ mode. (In reality, the switch is a transistor.)







simulate this circuit &ndash; Schematic created using CircuitLab

So it is not possible to output anything in HiZ mode; in HiZ mode, the chip does not (and cannot) affect the line's voltage.