Is it possible you can get meltan in ultramoon or transfer into ultramoon. On the website, it said it's tradeable?
You cannot obtain Meltan in Ultra Sun/Ultra Moon through any method. Meltan can be obtained in LGP/E via Pokemon Go and Sword/Shield via Pokemon Home transfer.
"Trade/migrate from another game" means the Pokémon cannot be obtained in that game or it means the Pokémon is an event-exclusive Pokémon. Also if a Pokémon is a version-exclusive Pokémon (example: Alolan Vulpix is only available in Ultra Sun, but Alolan Sandsrew is only available in Ultra Moon), then "Trade/migrate from another game" is possible.
Unfortunately you can't