Yes but the author is American So he'll always have to pay US taxes even if he moves out of the US.(I'm aware about tax treaties but in most cases they still have to pay the difference)
It is true depending on the country he lives in. As he's a US citizen, even if he lives overseas he needs to fill in a W-9 form to get the money from GitHub.
He can spend $300/month in many parts of Europe, get good healthcare. Even with paying some taxes to support the US billionaires, he'll live a very good life. In SV for that money he'd have to climb over zombies to go get the groceries
It may depend on what country you're in, but in the US if you're just some rando, "donations" are still taxable income. The form GitHub sends you for filling purposes is the same one used for contractors.
If the earnings are made outside of the US then he doesn't pay tax on less than 100k. The earnings are in the US so he would need to pay, he still has to fill in a W9 form as he's a US citizen.
