It's a weird mix of buddhist "I was and am at peace with this", but with a "I feel like I missed out" mixed in. But he fully admits it's his own fault because he was up in his own world, instead of leaving the business end to his agent.
But I don't believe this would stand up in court. There was a verbal agreement before he signed a contract. He was paid for his one-off contribution; I don't know how much work he delivered or how many hours he put into it, but 20K is half a year's income for some people, not to be sniffed at.
This has come up recently as well in some drama about voice acting on the one hand, and video game soundtracks on the other. Normally you get paid for the time spent, or the amount of your contributions used. But in the case of the voice actor, they decided the normal amount - or as it turned out later, a chunk well above the normal amount - wasn't enough and they threw it onto the internet.
But in the end, I think this is just a business transaction - one part of one part of the game, paid for with a five figure sum. The author neglected to do his due diligence and neglected to review and sign the contract in a timely fashion. Mojang and Microsoft may be susceptible to a lawsuit because the author never surrendered the rights to his work until now, but his unwillingness to cooperate while accepting payment for work delivered won't work in his favor.
And while he gave the text up into the public domain, I don't know if this holds up in court; iirc, copyright law only applies if you defend your copyright. Which means that if the author's copyright is voided, then Microsoft's copyright for Minecraft and all it contains might take over, and they might sue him instead. I'm not a lawyer or versed in copyright law, so this is just fantasizing.