I kinda feel like this guy is overstating his contribution to Minecraft. It's nice to have an end screen message, but that isn't what drove the value of the game and could have easily been replaced by something else. Rule #1 of getting paid for your work is don't do a bunch of work without signing a contract first. Even if you're an artist. I highly doubt he could have released that poem on his own and turned it into…
I'm fairly sure that no credible lawyer would give him the time of the day if he did try to get more now. He did work, he accepted money for that work and only later he thinks he's entitled to more and wants to re-negotiate.
I don't know about his jurisdiction, but here even as a layman I'm confident that doing the work and accepting the money would be seen as acceptance:
"As well as using words, a contract could be implied by conduct of the parties, for example, by jumping into a black cab and stating your destination, this conduct would be taken as an agreement that the taxi driver will take you to your destination and that you will pay a price for it."
https://www.legislation.gov.uk/ukpga/2015/15/notes/division/...
The idea that just because there isn't a written contract that he magically retains the rights is magical thinking and I'm surprised that no-one he's previously told this story to has pointed that out to him.