I am not too familiar with Windows development in the 16-bit era, so I do not understand what Raiter means when he writes that he “de-Petzoldized” the code. I thought Petzold’s work on documenting the Windows API was foundational. Was it the style?

Yes, Petzold's examples used a certain style to illustrate the use of the Windows API as clearly as possible. Petzold programs would have a giant switch statement handling WndProc messages, state would be held in tons of global variables, and everything crammed in to one huge file. Great for showing a concept and demonstrating how something works, not so good for developing a maintainable, reliable piece of software.