I've been watching this trend toward SDD. Makes sense but it feels like the process pendulum is swinging back toward the pre-agile era of functional specs and design documents. Not quite Big Design Up Front[0] but maybe increasingly working software == comprehensive documentation[1]?
Waterfall anyone?!
[0] https://en.wikipedia.org/wiki/Big_design_up_front
[1] https://agilemanifesto.org/