Academics might argue on the specifics, but generally speaking, where else could the German-speaking Goths come from, if not from the area inhabited by Germans? At worst, the area of German ethnogenesis could be slightly revised to include not only Southern Sweden but also the mouth of Vistula, which is not exactly half the world away. And IIRC Wielbark ended up disappearing pretty much completely at some point between the establishment of the Gothic kingdom and the migration of Slavs, so I don't find it particularly strange. It's still a migration, it's just that some "stragglers" were still around in the Vistula area while the "vanguard" already established the Cherniachov polity on the lower Dnieper.