Were you able to find a solution or at least an answer to this? Apple's official guide on App Search shows a snippet comparing the activityType with a custom one, but I too keep getting "com.apple.corespotlightitem" despite initializing my activities with my own type.
Did you add the NSUserActivityTypes array of strings to the Info.plist file?
See here... scroll down to "Activity Types" section for an example.
I know it's several months since you asked, but I just discovered this today.
"com.apple.corespotlightitem" = CSSearchableItemActionType - which is found in CSSeachableItem.h
The activity has a "userInfo" property, a Dictionary. In my case, it has a single key - CSSearchableItemActivityIdentifier, which is the uniqueIdentifier of the CSSearchableItem that I created when I indexed it, originally.