>That’s part of what waterfall advocates for. Write a spec, and decompose to tasks until you can implement each piece in code.

That's what agile advocates for too. The difference is purely in how much spec you write before you start implementing.

Waterfall says specify the whole milestone up front before developing. Agile says create the minimum viable spec before implementing and then getting back to iterating on the spec again straight after putting it into a customer's hands.

Waterfall doesnt really get a bad rap it doesnt deserve. The longer those feedback loops are the more scope you have for fucking up and not dealing with it quickly enough.