OP here. I wouldn't necessarily call it a waterfall, but it's definitely systemized. The main idea was to remove the vibe from vibe coding and use the AI as a tool rather than as the developer itself. By starting off with knowing exactly what we want to develop on a high(ish) level (= PRD), we can then create an implementation plan (epic) and break it down into action items (tasks/issues).
One of the benefits of using AI is that these processes, which I personally never followed in the pre-AI era, are now easy and frictionless to implement.
I think for me personally, such a linear breakdown of the design process doesn't work. I might write down "I want to do X, which I think can be accomplished with design Y, which can be broken down into tasks A, B, and C" but after implementing A I realize I actually want X' or need to evolve the design to Y' or that a better next task is actually D which I didn't think of before.