The original Wii U gets wiped AND the new Wii U also gets wiped, I imagine? Or the saves get merged on the new console?
Took me a couple readings to figure it out. The info in the OP relates to cases where you already have two Wii Us with separate NNIDs and separate game data and save data on them, i.e. neither Wii Us are new.
In that case the account data, game data and save data from the source console
replaces the account data, game data and save data on the target console. Preexisting saves on the target console are lost forever, but pre existing NNIDs can be relinked after the transfer is complete which allows you to redownload pre existing games that were on the target console.
Unlike Wii U data, Wii data on the target console is
not written over but merged with Wii data from the source console.
After all that is done, all data on the source console is deleted.
Tl;dr: Best to transfer from an old Wii U to a new out-of-the-box Wii U that hasn't been used yet, in order to not lose saves on the new Wii U.