Which is an implied contract, yes. Which a court would probably find that Mojang have an implied license to use the poem, yes.
But the terms of that license are unknown - a court may well decide that it did not apply to Microsoft since he didn't sign that contract. And it also means that he did not assign copyright to Mojang or Microsoft which removes that avenue of them claiming they can use it for free.
But the terms of that license are unknown - a court may well decide that it did not apply to Microsoft since he didn't sign that contract. And it also means that he did not assign copyright to Mojang or Microsoft which removes that avenue of them claiming they can use it for free.