What the linked article is trying to say is that not every I2C device will "pull" the bus down the same as others, and goes on to exaggerate this by intentionally weakening one of the devices with a series resistor. Trace resistance and drive strength will determine just how low a logic low really is, as you can see from the picture on that page:
The author of the page has placed a 200 ohm resistor in series with the SDA line and is measuring the voltage at the slave. Since the master has been made weaker, it won't pull to ground but rather to a small positive voltage (which is still low enough to be seen as a logic 0 by the slave). Thus with a scope you can see that the device that is driving all the way down to zero is the slave (since it does not have an additional 200 ohms of resistance between its driver and ground), and the device that is pulling almost to ground is the master.
I used a similar technique when I was debugging the Nintendo 64 controller protocol. It's a one-wire bidirectional protocol and I wanted to know what each side was transmitting. I placed a 330 ohm resistor in the data line and measured across the resistor. Positive pulses were the master talking to the slave, and negative pulses were the slave talking to the master. You could do the same here, as long as your oscilloscope's ground lead was not tied back to earth ground (or if you used a differential probe).