I agree with other commenters that the author is greedy and petty. He accepted €20,000 for his work, a decent amount. Then he decided that he should make way more money because the game was successful. Then he tried to take advantage of the fact that he hadn't signed a contract, even though he had obviously accepted the terms when he accepted the money. He behaved shittily by not honoring an informal contract. He tried to paint it as some huge company legal dept vs little guy thing based on the fact that it's now Microsoft and yes, work for hire contracts drafted by corporate lawyers do look evil. But he had already reneged on his handshake agreement, after spending the 20K, even before big companies or professional lawyers were involved.
Now he's trying to parlay his brush with fame into Substack subscriptions. Good luck to him.