To my knowledge, that has never been claimed.
Waterfall is what many of us old fogies in the industry experienced as the "defacto" methodology for a long time. It made intuitive sense that in order to design a build a project that you would first, you know, DESIGN it. Then you'd kick over that design to software developers that were expected to implement it.
Iteration in the design and development process, the idea of "people before process" and getting designers and engineers to collaborate early on etc. was not obvious. That's where all of Agile's "waterfall" talk came from. The fact that for a long time what companies were doing, while never exactly the same process as each other, was always waterfall-like because that's what made the most sense in an industry that was very new and in which no one knew wtf they were doing... so they took knowledge from other domains and tried to make it fit. That's a large part of what Fred Brook's The Mythical Man Month talks about.
It's only now that a new generation of developers has come up in a world where all they've ever known was "Agile" and "Scrum", that the world they know is so far removed from the "non-Agile" world that these books describe.
A colleague of mine the other day was talking about experimenting with something using the browser's `postMessage` API 8 years ago. My initial reaction was "did postMessage exist 8 years ago?" And then I remembered that 8 years ago was 2016 and it's already 2024. Many "experienced" people coding today have 5 years experience... and then they talk about concepts that were a reaction to how things were being done in the 80s and 90s as if those decades never happened ... because if they had even been born yet they were still children, so they weren't there to live that reality and the pain that what came later was a reaction to.