But at the same time it's pretty clear he was jealous of all the money he didn't want to talk about, and that he saw himself as a partner rather than a contractor.
It is what it is, everyone involved probably learned some form of lesson, and I don't see why we need to create villains or victims in this story.
More fundamentally, it takes two to tango. Just because the perspective of Mojang is the more common one in the industry doesn't make it the more "correct" interpretation. It's very much a two-way street and it's on both parties to make sure they understand each other when things are not explicitly spelled out and agreed upon.
Mojang, at a minimum, paid for a license to use the poem in the game. They wanted to have this on paper because it would make the deal with Microsoft easier, but they would have been right about that fact either way. The author, on the other hand, believes they are in a situation where Microsoft has no right to use the End Poem despite the fact they accepted $20k for it.
I mean, ignore all the stuff about friends, blah blah, Carl has apparently signed them up for whatever the defaults are under (based on other comments) UK IP laws?
But the reality is that _all_ of the misunderstanding comes from him. There is no "mutual misunderstanding", Mojang understood the conversation perfectly well.
The author asked for an offer, Mojang offered $20k, and the author accepted. It's not reasonable to expect Mojang to "understand" the author is going to hold some grudge about believing they deserve more than the offer they accepted without negotiation.
The correct response from the author would be something along the lines of "I am entirely at fault, all of the misunderstanding was on my side, Mojang/Notch/Carl did nothing wrong".
Instead the author reminds us at every turn about how Notch gave bonuses to all the Mojang employees but not him, how they're such a great guy for not getting lawyers involved, and how they're turning their work into a "priceless gift" (despite being paid $20000 for it).