Where in the rules does it say that an attack which triggers mortal wounds is non-attack damage? It is generated through an attack profile. Things like Stormcast Lanterns and other weapons which do not have to hit or wound are not attacks. But, if it triggers off an attack roll, being either a hit or wound roll, then it is obviously an attack based upon how attacking is defined in the core rules of AoS.
"MORTAL WOUNDS
Some attacks, spells and abilities inflict mortal wounds. Do not make hit, wound or save rolls for mortal wounds. Instead, the damage inflicted on the target is equal to the number of mortal wounds that were suffered. Allocate any mortal wounds that are caused while a unit is attacking at the same time as any other wounds caused by the unit’s attacks, after all of the unit’s attacks have been completed. Mortal wounds caused at other times are allocated to models in the target unit as soon as they occur, in the same manner as wounds caused by damage from an attack. After they have been allocated, a mortal wound is treated in the same manner as any other wound for all rules purposes."
Clearly states that attacks can deal MWs.