There are other cases, though, where the courts have declined to imply transfer of copyright or license - even with a written contract - when it hasn't been explicitly specified.

cf https://www.albright-ip.co.uk/2013/04/implied-assignments/ for one example

> The idea that just because there isn't a written contract that he magically retains the rights is magical thinking

IANAL, obvs., but I think the situation would be "the implied contract would imply a license to use the poem but not transfer of copyright" - the key point would be whether that license was "perpetual, exclusive" to Mojang. If not perpetual, he could revoke it at any time and Microsoft will then have a copyright problem for any copy sold after that point. If exclusive, the license may not have transferred to Microsoft in which case they have an extremely large problem given the number of copies sold since the buyout.