To me, that doesn't make sense. It would be like if someone burned to death in a vacant home with no obvious entry, and they didn't bother to investigate the home and instead just said, oh it was a fire. Wouldn't the chimney have everything to do with the investigation? Maybe the investigation is completed already, but surely it would have included an analysis of what exactly happened, including the boy's positioning and whether there were signs inside the chimney that he'd tried to escape, etc? Whether all of the things noted in the preliminary autopsy report make sense as far as the 'scene' (inside the chimney), etc? How and when he could've tried to put his jacket through the flue opening, etc? How long he could have survived inside there, considering the available oxygen, etc? Confirming how he got up there, and that he did in fact go up there (backed with evidence like shoeprints or handprints on the antenna), etc? imo.
Yes, I'm sure their investigation of the chimney, home, etc. is done. I'm not sure they would reveal non-essential details like whether he'd tried to escape, for example. We may want to know those details, but I'm sure it was fairly obvious to the people who found the body exactly what happened.