Agile is (to the extent it has concrete substance) very similar to Lean, which (while it has come to software) actually started in manufacturing and engineering of tangible products. The difference between the two is mainly that Lean has a much stronger culture (and has thus also produced a lot more tools) around validation of methods through measurement, and thus is a lot more woo-resistant than Agile, which has proven to be decidedly prone to devolving in exactly the same kind of one-size fits all, consultant-pushed, top-down methodologies without good feedback on what works in the particular environment that the Agile Manifesto was a response to.

> You can't build a house or a plane using agile. It has to be waterfall.

"Agile" and "waterfall" aren't opposed, agile is opposed to the idea that any one methodology -- Scrum as much as Waterfall -- can be selected as right for a team without reference to the particular team and context in which they are working. To the extent that it works in a particular context, waterfall can be Agile.