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.

MS can now use the End Poem in spin off products, or even adapt it and use variations or sequels of it in spin off products. Pretty good deal for them, I'd say.

Though to be perfectly honest, if I were them, and they needed something like that, I'd hire Julian to write it. Or maybe ask him to write it for free after giving him a massive donation.