Walz also campaigned as a liberal. So did Patrick Murphy and basically every seat that we picked up in the Northeast.
Pat Murphy didn't campaign as a liberal and even if he did, that's not the point. He won because he was a young Iraq vet and his supporters will tell you the same. Just as I started worrying that we might not get that seat back, I learned that the Dems are very worried about losing it (one prominent activist is already conceding the seat).
Anyway, you picked up a good deal of those seats by campaigning as liberals but not "basically every seat." You certainly didn't get PA 4 or 10 because your candidates ran as liberals. Same with PA 8. That leaves you with a handful of other pickups in the NE and even with those pickups, you needed moderates in the rest of the country to win the majority that you now have.