Indirect kill of a leader gives no loot
The kill_unit() procedure which is responsible for killing side effects gives loot for a killed gameloss unit if ruleset sets so but only if the unit was the one which actually has been defending. I.e., if you play with killstack on and your leader is in a vulnerable stack covered by a better defender and that defender loses, then the leader is just gone with the stack at the end of the function. His civilisation in effect disappears or goes barbarian but the victor gets no cities, gold or map that they would have if the leader were killed directly. Yes, that goes in line with what happens with a barbarian leader, but it's probably not intended so and is a bug.
Well, when a leader is nuked, maybe we should actually get no loot... or it should be configured in later versions.