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.
IANAL but AFAIK this is the case also in Sweden. (Researched it when I (otherwise an amateur) was asked to do paid photography for someone.)
In fact, though now my memory is hazy and so I speculate more wildly, not even most kinds of employment grants copyright to the employer. Exceptions being software developers and teachers, I think.
> do not automatically assign an exclusive copyright to the buyer
Of course, because in germany you can not transfer your copyright at all, unless you die. ;)
Without being nitpicky you are still right in the way you meant it: There is no automatic usage right afaik. (And also google results agree: eg https://likvi.de/blog/urheberrecht )
If you want to nitpick, Germany technically doesn't have copyright in the US sense and instead the term is usually considered equivalent to what is called Urheberrecht (author's rights), which is indeed not transferrable.
You can however grant an unlimited, transferrable, exclusive license, which for all intents and purposes is like "transferring copyright".
Your nitpick isn't entirely irrelevant though. This is why Germany does not recognize public domain dedications and why CC0 is necessary.
I think that in civil law countries you can sometimes give / sell away your copy rights, but never your moral rights ?
(Though copy rights themselves might also be treated as actual rights, rather than a temporary monopoly, an exception to the normal state of everything being in the public domain that prevails in the common law countries ?)
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.