As you said, who knows how the 7 stereo back speakers are being matrixed.
No mystery how the additional channels are generated in the 5/7 stereo modes. The fronts, sides and rears are each treated as stereo pairs: all left side speakers get left channel info (above their crossover points); ditto the right side. The centre channel and subwoofer output each get a simple L+R summed signal (again, defined by their crossover settings).
This has traditionally been how every 5/7 stereo mode (sometimes called "party" mode) has worked. The only variation I've ever seen is where the surround volume is slightly lower compared front channels, in order to keep the energy somewhat foward focused. Otherwise there's no surround processing, such as matrix decoding, taking place.