Why would this logic gate use negative voltages?
ECL outputs are referenced to the most positive supply rail.  This means that any noise appearing on the most positive supply rail will be directly coupled onto the output signal.  For example, if the power supply is 5V and GND, then all outputs would be referenced to 5V, and any noise on the 5V supply would also be seen at the ECL outputs.  Therefore, the older literature calls for ECL chips to be powered by a negative power supply, such as -5V and GND.  By using GND as the most positive supply rail, it is easier to maintain cleaner signals at the ECL outputs.  GND is generally found to be less noisy as compared to a non-GND potential.  This does not mean it is impossible to use a positive power supply.  It does mean that precautions must be taken to ensure very little noise is coupled onto the most positive supply rail in order to maintain clean outputs