what does the option creative inventory do?

It does nothing at the moment. I think I added the option with the intent to add some feature and then forgot about it :)
I uploaded the mod after having it just locally for months or more so don't really remember. Maybe it was supposed to set all partial stacks to full stacks for infinite items?

👍

I actually already implemented this I see now, looking through the code. But it's just always disabled without reading the setting. I might enable it, but then I also need to make some admin-only setting if this should be allowed for players (non-admins) to use. Those using this mod in multiplayer might want players to get infinite inventory slots but don't want to give infinite items.