They ultimately did, though state TV needed to include it in their cooking show as otherwise the fish would've rotted on the shelves -- the cans had Russian labels and while people learned Russian in school that doesn't mean that you know the name of a random fish. People didn't know what it was, what to do with it, so they didn't buy it.
Also you might've noticed that the GDR had a coastline.
In any case, and you seem to be trying hard to not address it: The point is not the fish. And it wasn't always fish, but other random shit. The point is that the GDR didn't have a choice. And the Russians didn't even have the decency to ship the cans with German labels. Probably were sitting in some warehouse collecting dust in Russia in the fist place.