Mining profitability depends on the hardware model, price of electricity, price of bitcoin, and the number of miners on the network.
This seems to be a mistake. The profitability of mining isn't influenced by the number of miners, but rather by the difficulty.
It only makes sense if the number of miners varies the hashrate, which in turn increases/decreases the difficulty, which in turn makes mining cheaper/more expensive. But I agree that the number of miners alone does not make mining profitable, I think the author was referring to hashrate.
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