I've seen the BTU charts on this stuff, and I'm starting to think there is a BIG difference between a yard walnut and one in a timber stand. I just cut up an old (not SUPER old, but old) yard walnut and it was lightweight and split VERY easily. The Black Walnut trees that I have been burning from our timber are equally as old (by a ring count) but are 2/3 the diameter and about 50% taller. They burn much better. Coaling characteristics of even the timber walnut is what you describe, but I feel that it ranks higher than most of the BTU charts I've seen, so It's one of my favorite woods. The BW from my timber seasons quickly, is easy to split, and burns well.
I did find BTU chart that was from some (memory is fuzzy) midwestern university, and that one had it listed much closer to what I tend to see, so I also wonder if there is a regional difference in walnut.