That stat is effectively impossible to nail down after the fact. Air pollution is one metric of am extremely complex system, and coal or any other energy source is one of countless inputs impacting the environment simultaneously. At best we can design and run models to help get clues on what impact any one input has, but those models will always be rough hints at correlation with results that are heavily influenced by the assumptions used when designing the model.

You are correct that any studies claiming to put a number on how many deaths should be attributed to Chernobyl is propaganda. That holds true for the rest as well though, including any claims of exactly how many have died due to coal or cow farts for example.