Mojang yes. Microsoft... eh... that's more difficult and depends on a number of factors. As I understand it, Mojang was acquired by Microsoft, i.e. Microsoft inherits all contracts Mojang held. IANAL but in this case they would probably be able to use the poem under the same implied limited license.
But I'm not sure how this works when Minecraft itself was moved out of Mojang in any way (i.e. if this is formally a Microsoft game and no longer a Mojang, now subsidiary of Microsoft, game).
Anyhow, the CC0 license also applies to Microsoft going forward so unless he wants to challenge their rights prior to this, Microsoft is apparently in the clear now. They just can't sue anyone who wants to also use the poem because they don't have standing.
But I'm not sure how this works when Minecraft itself was moved out of Mojang in any way (i.e. if this is formally a Microsoft game and no longer a Mojang, now subsidiary of Microsoft, game).
Anyhow, the CC0 license also applies to Microsoft going forward so unless he wants to challenge their rights prior to this, Microsoft is apparently in the clear now. They just can't sue anyone who wants to also use the poem because they don't have standing.