the assumption that real miners couldn't (or wouldn't) have written a song like that

I don't think that at all. (My grandfather was a miner, for what it's worth.) But we know that Lloyd did a particular kind of job on some old songs - improving rhymes, sharpening political points & generally turning a dull old song into a catchy and memorable one. We also know that he sometimes lied about what he was doing and invented singers who he had supposedly collected his version of the song from.

The Recruited Collier was
...supposedly collected by Lloyd from a named singer
...never found anywhere else in traditional singers' repertoires
...similar to a poem called "Jenny's Complaint", but sharper and punchier
...and was actually adapted by Lloyd from "Jenny's Complaint"

The Blackleg Miner was
...supposedly collected by Lloyd from a named singer
...never found anywhere else in traditional singers' repertoires
...similar to a song called The Yahie Miners, but sharper and punchier

...and?

We don't know for certain that TBM was one of Bert's, but the circumstantial evidence (and absence of evidence) is quite strong.