I don't know about Swedish law but in German law works created outside of an employment contract do not automatically assign an exclusive copyright to the buyer, so at best Mojang had a non-exclusive, non-transferrable, limited usage right for the poem for the use in Minecraft and nothing else. As I understand it, this makes his claim that Microsoft may not have the right to use it questionable but it confirms his main point that they don't own it. Given that the author is German, this seems relevant.

For what it's worth, this often comes as a surprise even to many German creators who are not aware of this because intuitively if you get paid to create something "for" someone, you sell them your rights to it. But in reality that requires an explicit contract spelling this out. A typical example is professional photographers actually only selling you prints of the photos they take for you, or a design agency not giving you the source files for designs they create for you.