Have you ever seen water jump back up a few stages of a waterfall? (Hint: no.) It's called the Waterfall model exactly because that kind of thing is explicitly not part of it (see Figure 2 in the Royce paper linked above). That's the whole point of the name.
It goes without saying that no software development effort has ever lived up to this standard. Nonetheless, the fact that it is not possible to develop non-trivial designs (for software or anything else) like this in no way prevented people from advocating it as the "ideal" design process.
The first formal description of the waterfall model is often cited as a 1970 article by Winston W. Royce,[4][5] although Royce did not use the term "waterfall" in this article. Royce presented this model as an example of a flawed, non-working model.[6] This, in fact, is how the term is generally used in writing about software development—to describe a critical view of a commonly used software development practice.
You're being a bit too literal. The name came from the resemblance of a diagram in the paper to a waterfall. The name wasn't meant to constrain the methodology to strict characteristics of a waterfall. The steps are meant to be followed in order. It is gated. But, nothing constrains the steps from being repeated. That is the crux of the difference between Dr. Royce's paper and what the military formed as a standard based on the paper.
I took a college comp sci course a few years ago. The professor talked for a few minutes about the Waterfall methodology. In short, she said "we in academia gaffed by promoting the Waterfall methodology for several decades. It's now cause for laughter in academic circles when someone proposes the Waterfall method for software development."
It goes without saying that no software development effort has ever lived up to this standard. Nonetheless, the fact that it is not possible to develop non-trivial designs (for software or anything else) like this in no way prevented people from advocating it as the "ideal" design process.