Is it true that overtraining does not exist but its instead just under-recovery as to not rest,sleep,drink and eat enough?
Let's be specific: by "overtraining", we really mean "applying a stress from which the organism cannot recover and adapt" in the context of the General Adaptation Syndrome, which we can summarize here as "stress, recovery, and adaptation".
So, the question becomes: "Is it possible to stress oneself (that is, volitionally) to an unrecoverable extent?" I'm unsure whether this is possible. The accounts of Pheidippides suggest that yes, it is possible. I suspect that other examples exist.
On the other hand, the vast majority of us probably aren't "overtrained".