It sounds like an awful, muddy mess that would take a great deal of time and money to litigate if the author decided to take that path. Without an explicit contract, even determining which court (and which nation) has jurisdiction for this kind of international transaction sounds like a nightmare.
I agree that in most jurisdictions I have any familiarity with, some kind of contract likely exists for usage of the text, but that is a fundamentally different question from ownership of the text. And I imagine that the text has subsequently been used by Mojang and Microsoft for other purposes, which may or may not be found to be covered by the original agreement.
In short, it's exactly the kind of ambiguity that big corporations hate when it comes to intellectual property.