So it appears that autothrottle was off "A/THR déconnectée" and has "disengaged" on its own and that it is not considered as a "serious incident" by BEA but just as an incident.
Indeed, A/THR disconnect is not a serious incident at all.
But a flight crew who allows the speed to drop almost to stall speed causes surely a "serious incident" IMHO.
BTW
AB calls it auto-thrust, not autothrottle.