If Walther says that the new production Walther PPK/S grips are interchangeable with S&W PPK/S grips, then you can take their word for it. Walther arms bought all of the leftover parts and tooling from S&W after their production run was concluded, ergo the frames on the new PPK/S were most likely built to the same specifications using the same equipment, and may even have been fitted with leftover grips from the S&W grips.
So yeah, don't waste your money on a cheap pair of grips, just buy some from Altamont. My S&W PPK/S is fitted with Altamont Super Rosewood Grips, and they were worth every penny.