The WPC-Design document reports that: "Transmitter use a differential bi-phase encoding scheme to modulate data bits in the Power Signal."
which encoding method do they mean? - Differential Manchester encoding?
From a communications standpoint, bi-phase coding means that the signal has two states that are distinguished only by the phase of the signal. The amplitude is constant. See binary phase-shift keying (BPSK or 2-PSK):
Differential BPSK (DBPSK) means that the difference of the current bits phase and the phase of the previous bit is taken. This is helpful when the absolute phase is not known and the phase change to the previous bit is used instead. https://en.wikipedia.org/wiki/Differential_coding
"Transmitter use a differential bi-phase encoding scheme to modulate data bits in the Power Signal."
does not state that a differential manchester code is used, but this type of code has the above mentioned properties.