As another commenter noted, his recounts are magical realism but I don't believe we can completely discount his experience as made-up imaginary fiction. He probably had experiences similar to those he shared. His recounts were polished for publication, as they should be for the sake of readers.

The heart of this debate is whether or not the mystical experiences were rooted in ontological reality, or if it was a complete or partial hallucination. Modern neuroscience posits that all reality is a brain induced hallucination from moment to moment. So much of what we experience is filled in by the brain with anticipation, which is why we can dream and daydream even when no sensory stimulation is providing a substrate from ontological reality or from what's "out there."

In that regard, Carlos Casteneda could be recounting reality exactly as he experienced it. It doesn't mean his experience matches an ontologically objective reality. So the debate over what's real can depend on how you define "real."

The book could be a direct recount of Carlos Casteneda's reality without reflecting anything any scientist would call objective reality. Which one is real?

Most people instinctively side with the scientists, but since all experience is mediated by our senses and brain, how could anyone know what objective reality is? Carlos Casteneda's subjective experience, his self-created reality, is not more or less real than anyone else's.