Earlier quoted context omitted.
My interpretation of the legal status is that the author is entitled to accept the €20k as payment for his time but that amount doesn't constitute payment for the copyright of the work. Mojang subsequently wanted to purchase that copyright and hence why they sent the contract for £1 to be signed. Julian believed the copyright was worth more and so rejected the offer. All that is to say that if my interpretation is co…
At minimum, they are buying a non-exclusive license to copy the work. Beyond that is speculation. (Disclaimer: I work at Microsoft on nothing related, and love Minecraft the game.)
IANAL but I think Microsoft is in the clear here because they bought out Mojang but they couldn't for example use the ending (or derivative works of it) in a spin-off game because Mojang couldn't have done that either and they have no standing to go after anyone reproducing the poem as a whole or in part.
In fact, the poem now being CC0 grants Microsoft more rights than they had, except it's still non-exclusive, meaning the same rights are granted to everyone else too.