No, the problem was that any hero that is a core file AND is also redefined in a mod file does NOT receive any change to <DividesBattleExp> tag.
I found this out by editing Heavenfall's Hero file and not having any hero files of my own, and it still didn't work.
You can change the stats, you can change the traits, but the <DividesBattleExp> tag is not changed when you redefine it from the mods folder.
HOWEVER, completely new heroes can be defined how you want from the mods folder. As I have said, they are essentially the same heroes but with different InternalNames (the game sees them as different heroes).
The CoreQuest and CoreRandomEvents files are also changed in the core folders to reference the new mirror heroes (you can't edit quests and events from the mods folder).
The core file edits ARE required if you are to receive the NoXPSplit ability for existing, core heroes.