Your theory doesn't explain why they don't retrain in welding, plumbing, electrical work, construction, vehicle maintenance, etc which are all more similar to mining (particularly within certain parts of each field) than they are office work.

At least in my state, many of the (re)training programs are for those and consist of a couple weeks of lecture followed by a few months of apprenticeship -- I suspect that they're simply sticking with the familiar while there's hope. People do that.

The empty classrooms is probably referring to the fact that every program begins with a little domain specific knowledge, so the people observing the intake look at that part of the funnel that every program goes through -- and then make comments about it from their perspective.

The article is a little deaf in saying "100 programs from coding to nursing". I'm guessing the fields are actually widely broader than that in the kind of work, since he found a program in coal mining among those 100, and I don't consider that between coding and nursing in any sense. (I think that left you with the impression they only train office or service work, for instance.)