> a contract could be implied by conduct of the parties
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.
> 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.
> Microsoft bought Mojang not specific property of Mojang.
[1] says "When licences are acquired as part of an asset transaction, the contracts are taken over. In that case, the selling and buying parties need the cooperation of the licensor. This cooperation can take place before or possibly after the takeover. If the licence is not transferable, there will in principle not be a legally valid transfer unless the IP right holder in question gives his consent to the transfer of the licence."
Do you have a link to suggest that these definitely-are-lawyers are wrong in this interpretation?
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.