I was left wondering whether the sheriff simply claimed to have found some of the kids' bones or they actually did find them and didn't want to tell the author any real details.

I think there's probably a strong motivation for the sheriff's office to close the case, especially after so many years and so many resources expended. On top of that, there was probably some desire for them to make the author go away. Law enforcement officers have to protect the perception that they are both competent and caring, and this "enthusiast" proved to be better on both counts in this case. So either there's jealousy that makes them not give him the information that would have satisfied his curiosity (and spared him a couple more trips to the desert), or they just told him they found more bones when they actually hadn't to make him stop looking.

Either way, I think they owed him more than that for solving their cold case, but couldn't stand to do that because now people like me say things like "he solved their cold case."

Edit: Keep in mind this is the same office that had the responsibility to find all the bodies Charles Manson and gang buried out by the Barker Ranch, and didn't. I get that they have limited resources, so not blaming them, but it probably stings when this guy finished the job they started.