Machinbird,
A bit of study on the Vmo/Mmo protection mode indicates that activation of this protection will disengage autopilot as it pitches the aircraft up. This means, that the soonest such an event could have occurred would be at the very beginning of the ACARS sequence, thus perhaps explaining this message at the very beginning:
2:10:10WRN/WN0906010210 221002006AUTO FLT AP OFF
To me, if the AP disconnect is the consequence of the overspeed protection, alongside the AUTO FLT AP OFF WRN ACARS MSG, we should have got an OVERSPEED WRN ACARS MSG as well.