Agreed.
Marx's working class consists of those who do not own means of production, and are therefore compelled to sell their labour.
Miners do not lack means of production; they operate expensive mining machinery. That machinery continues to run, and earn money for the miner, even while the miner sits on the beach. Selling their labour really isn't a good model for what's going on. Miners are just ordinary capitalists, exploiting a market opportunity.