Bloodbath is like Rends cast by an entirely different character (who also has Lamentation equipped).
So you can get 2 stacks of Rend via Ambo or hard-cast, and 2 more from Bloodbath.
I think you might be right that this isn’t explicitly stated in the guide, but it is available in a link through the Build Mechanics section, which takes you to this post: Rend mechanics 2.6.7 - #3 by sVr-21743