How exactly should I understand the term "accidental hull loss"?
Both the MetroJet and GermanWings incidents were deliberate acts of destruction, not accidents in any common understanding of the word, or indeed in the sense defined by the Convention on International Civil Aviation.

The Chicago Convention, as it's also known, defines an accident as:


  An occurrence associated with the operation of an aircraft [...] in which  [...] a person is fatally or seriously injured [...] except when the injuries are [...] inflicted by other persons.


My emphasis added - see Annexe 13 of the Convention for the full definition - it's quite dry but interesting, if you're interested in that kind of thing. The exceptions it lists are illuminating.

Deliberate acts and "incidents"

I do think that the definitions in the Annexe are slightly odd. You'd think that the deliberate destruction of an airliner in flight should be called an incident, but an incident is apparently:


  An occurrence, other than an accident, [affecting safety].


I guess that's one way of describing a bombing or deliberate crash into terrain... but then there's also a Serious incident which is merely:


  An incident involving circumstances indicating that an accident nearly occurred. [my emphasis again]


So that would make the GermanWings and MetroJet cases incidents, but not serious incidents!

However: since "Appendix 7" of the ICAO Accident/Incident Reporting Manual lists "types of incident", and deliberate acts of destruction are not amongst them, I can only deduce that they are classified as neither incidents nor accidents. I am not sure exactly what the official terminology is for them.