I'm German and used to do freelance work. This interpretation is what I've been told by a lawyer in a similar case: the money is for the license, not the copyright, and unless specified, the license is the minimum of what could reasonably be expected, i.e. a non-exclusive, non-transferrable, limited license for the scope of the product.
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.