> If Agile can be summarized as "employ tighter feedback loops", the whole Agile thing was beyond useless.

Not just that, Royce's original paper that coined the term "waterfall" in 1970[1] can be summarized as "employ tighter feedback loops" compared to top-down design (figures 2-4 in the paper).

[1] https://www.praxisframework.org/files/royce1970.pdf