I asked a couple of people who have more recent experience with forensic burn evidence to comment on this case. Anthropologists typically deal with bones, as I'm sure everyone knows, and it's really hard to burn all bones in any kind of fire outside of specially designed crematoriums. For example, the base of the skull usually remains (and there's a bunch of research on how to begin to identify victims based on just that one part of the anatomy).
In this case, the melting point of any plastic tote (I'm assuming it's the heaviest kind of tote, the kind often seen in the truck beds of cowboy types) is not that high and unless gasoline was repeatedly applied, there would be remnants of the plastic.
The colleague who has the most experience said that if there was plastic, there had to be bone (unless someone fairly skilled had sifted through all the ashes prior to the investigator arriving). He pointed me to several citations involving very hot fires where police missed human fragments, but experienced forensic anthropologists found them. This is a specific subspecialty in forensic anthropology (burn fragments) but almost any biologically trained forensic anthropologist could pick out some of the major kinds of fragments that would remain. We have some plaster casts of bone fragments that survive that I can use in my own lab, and to be fair, almost no one would be able to tell that it was a burnt down piece of a skull. But after learning why, for example, the occiput burns the way it does, it's pretty easy to tell that one (and often overlooked).
Police in really big urban areas may have such a specialist somewhere within their own forensic team but most places do not have such a person on salary.
Someone like me would recognize some of the shapes of burned down bones (skull bones in particular) but only a very experienced and gifted forensic osteologist would get catch most of them; they almost always find things that police don't find but even they would want considerable forensic analysis of the site before it was touched). Further, this specialist would need to take all the ash, any baked pan, etc. to the lab.
So it's really hard to completely burn a human body. Lava flows don't always manage it. Jet plane crashes do not always manage it. It's also really hard to find and remove all of the bone.
Did they have a forensic archaeologist and osteologist, I wonder (sometimes the same person). They would have done a directed dig and bagging of the firebed and examined all fragments where instruments like powerful microscopes, x-ray and mri are available...as there are tell-tale signs embedded even in tiny bone fragments that reveal that they are bones. For example, I know how to tell wood or plastic from bone by the growth patterns visible only under a very good microscope (one that exceeds the budget of my college's lab). A person who is doing full time forensics in a well funded university lab, though, can see so much more. There are techniques (such as dusting with gold powder) to make these patterns more visible even on very small objects. Some parts of the skull can be used (even if badly burned) to determine age, sex and occasionally even a guess at ethnicity (used for reunification forensics).
So, a forensic anthropologist or osteologist would have been helpful to show that in fact, that burn area was used to burn a human. Further, with many fragments, the age of the person could be known. All of that would be helpful in prosecution, although this is apparently such a slam-dunk case that perhaps the State didn't want to pay for all of this??
There are some pretty darned good experts in Colorado, though, just saying.
