They're not assuming anything about what miners do with the coins once they mine them. They're just assuming mining is efficient — that the cost (inclusive of c

They're not assuming anything about what miners do with the coins once they mine them. They're just assuming mining is efficient — that the cost (inclusive of capital, energy, etc.) of mining is roughly equal to the benefit of mining, which is equal to the market price of bitcoin times the number of coins mined. (If you're planning to hodl, you can hodl either by mining or by buying on the open market, so an efficient market should ensure the cost of each is roughly equal.)