I’d still argue against that. I’ve had one true showerthought and it didn’t manifest as monologue, even though I do have an internal monologue. I had a concept and images for it. I spent some time trying to put it into words.
I still don’t see how a showerthought (or any thought) has to have a verbal origin in the thinker’s mind; I would argue any internal monologue is but a secondary step after a thought has occurred. I’ve never heard of anyone being unable to predict what their own internal monologue is saying, and I’ve never heard of anyone being unable to make quick decisions because they had to first hear a command in their minds.