I don't want to comment too much on Indo-European migrations seeing as that's really not my field but i think its much much more likely that when Tacitus said this what he was thinking about was things like Greek and Phoenician colonisation. Firstly we know that the Roman world was familiar with the fact that the Greeks and Phoenicians had established these colonies via sea around the Mediterranean and black sea worlds and the memory of connections between their homelands and Greece had been retained and written about over time.

There's also the fact that Tacitus is not referring to the PIE disruption in the text itself. The centre of his argument after all is that he doesn't think that the Germans couldn't have come from anywhere else because no one could migrate via the North Sea and the like and who would want to migrate to Germany anyway? So this along with other information would suggest Tacitus didn't know about any Indo-European migrations that occurred even for him in the distant past.

But his core statement that migration can only occur by land isn't really correct. We've got some solid counter examples from both earlier and later of land based migration movements. For example the Celtic entry into the Balkans an invasion. And according to Caesar during his Gallic wars he encountered the Helvetii, Usipetes and Tencteri trying to migrate by land.