Why are some item passives considered abilities?

Long story short, got salty and looked into why a Xerath ult hits minion -> luden's passive which hits me -> comet hits me

Since Comet says it can only be proc'd by abilities, this means that luden's echo passive is considered an ability. Why is this not the case with other items that have passives? Liandries passive will not proc Comet, same with Sunfire. Some active items will proc comet but only if they do damage (Bork, Gunblade) but if they do not do damage they will not (Randuins). So what is considered an ability in terms of comet? Since comet / ludens is so potent in the current meta, could this interaction be changed?