The Third Geneva Convention doesn't require someone to be part of the armed forces of a nation state or for their to be a declaration of war to be considered a lawful combatant. They can be part of a militia or insurget group.
From the convention:
"A. Prisoners of war, in the sense of the present Convention, are persons belonging to one of the following categories, who have fallen into the power of the enemy:
1. Members of the armed forces of a Party to the conflict as well as members of militias or volunteer corps forming part of such armed forces.
2. Members of other militias and members of other volunteer corps, including those of organized resistance movements, belonging to a Party to the conflict and operating in or outside their own territory, even if this territory is occupied, provided that such militias or volunteer corps, including such organized resistance movements, fulfil the following conditions:
(a) That of being commanded by a person responsible for his subordinates;
(b) That of having a fixed distinctive sign recognizable at a distance;
(c) That of carrying arms openly;
(d) That of conducting their operations in accordance with the laws and customs of war.
[...]
6. Inhabitants of a non-occupied territory, who on the approach of the enemy spontaneously take up arms to resist the invading forces, without having had time to form themselves into regular armed units, provided they carry arms openly and respect the laws and customs of war."
However, these people almost certainly violate one or more of these requirements. I have never heard of a suicide bomber openly carrying their bomb. They hide the bombs in vehicles or under clothing and use the trappings of civilians to get close to their target and detonate.
The US pilot mentioned above used his weapon as a suicide bomb, but be bore his arms openly. He was a member of the armed forces of a combatant to the war. He wore a uniform, he was part of a recognised rank structure receiving orders. As he attacked his target with his plane it was known that he was the enemy and the Japanese had the opportunity to openly confront him.
The scum in this article are recruiting people that would violate almost every possible requirement for being a lawful combatant. They violate the only really important requirement which is to carry arms openly and have some sort of mark distinguishing them as a combatant. The way they "fight" marks them as war criminals.
Italy needs to rewrite some laws. In the meantime they should be placed into military custody as POW's since the court has basically recognised them as lawful combatants.