Okay. So, honestly, look at the people who work the mines. Not the engineers or supervisors, but the bulk of the 9-5 guys. What percentage of them, in less than a year's worth of school/retraining, would be really good coders - the kind you could put on a project in an office in SF and expect to get a similar result to someone who's first chosen profession from their teens was coding and spent 4-6 years in post-secondary school learning the art, science, and math of coding?
I say this because Bloomberg is probably right. I'll bet at least 80% of them can't make up for lost time in a year or less (which, if you want to completely fund their retraining, including costs and living expenses is going to run towards $100k each). I'd bet more than 50% couldn't do it in 4 years. I say that because more than 50% of the general population wouldn't make it, and coal miners are no different.
As you said, coal miners are the product of their families, and often families where higher education is neither valued nor rewarded. It's not about better or worse, smart or stupid, it's about expectations and preparation. Take a 35 year old who hasn't done more than 3rd grade math in the last 20 years and put them into a math-intensive program. Most will fail miserably. Doesn't matter if they're a farmer, a coal miner, an automotive assembler, a construction worker, a retail cashier, or a salesman.
Bloomberg's words may feel like a put-down, but they're about as straight forward realistic as it gets. It's hard for "smart" people to understand that mere's applying yourself to a higher field of study often isn't enough to master it.