Of course it assumes each stat to be the sole source of income, for the purpose of answering this question: how many plays on Spotify would it take for the fees

Of course it assumes each stat to be the sole source of income, for the purpose of answering this question: how many plays on Spotify would it take for the fees to equate to one US minimum wage? That's all this is, and that's fine. It would be weird for the author to start factoring in other sources of income, because the point of this story is simply to make it easier to understand what "$0.00029 per play" means, by its ratio to a more tangible figure. I don't think the author is trying to imply that all those musicians on Spotify with fewer than 4m plays/month are actually earning less than minimum wage overall.