Is this slide wrong about the differential signal waveform?
The slide is correct, the signal is differential.
The voltage levels do not have to be symmetric like: -1 V, +1 V
Voltage levels like 0V, + 2 V are perfectly acceptable.
In the botton trace there is simply a DC voltage (of + 1V) added to the differential signal. It was added to both signals (the black and the blue) so the differential signal: (black - blue) remains the same!
Suppose you measured these voltages compared to a +1 V DC voltage: voltmeter between + 1 V DC and each signal. Then you would measure +1 V and -1 V.