Very unlikely. I'll explain, but it's a rather "clinical" explanation, and for that I'll apologize in advance.
Anesthesia professionals are experts at estimating blood loss in the OR, and especially blood loss that is diluted with irrigation fluid, amniotic fluid, etc, so bear with me. A waterslide is not the OR, granted-- but there are similarities when visually estimating blood loss, and calculating total blood volume based on age and weight.
Caleb was 10 years old, approximately 60 to 70 lbs, so had an estimated total blood volume of about 2 to 2 1/2 liters. IMO, from the amount of blood in the water visible in the pool in the aerial shots, plus the blood smears on the slide on the lower hill, pretty much his entire blood volume was in the water.
An "internal decapitation" is a hangman's fracture, C 1-2. Bloodless, and with the skin of the neck intact.
From what witnesses described, and from pictures of the slide and pool at the end, there was a great deal of blood lost, and lay people have described a "decapitation". They would not be describing this using these words if the "decapitation" was internal, and not visible. When a lay person uses that term, decapitation, they only mean one thing-- catastrophic separation, or nearly complete, separation of the head. (Apologies for being graphic.)
Again, apologies for sounding "clinical." In no way do I want to minimize the abject horror of what this poor child experienced, what the lifeguards, the witnesses, his family, the first responders all saw--and even the technicians who have to process the accident scene, and the maintenance workers who had to clean up the blood. And the medical examiner and workers at the morgue, also, though it's their job to process and document often horrific deaths. They will all be very deeply affected emotionally from what they saw. I have no doubt that some will have PTSD and nightmares. It's truly horrific. His poor family-- I have no idea how they will live with what happened to their dear sweet boy Caleb. I hope they can all find some kind of peace and comfort.