Author here. Happy to take any feedback about this post. I learned to write design docs at Microsoft and Google, and I thought they both had good culture around docs that hasn't percolated out as well as other engineering practices at those orgs. I haven't seen a thorough explanation of how to write design docs, so this is my attempt to externalize what I've learned about writing them.
I've found that developers usually like writing code and avoid contributing to documentation. For some, it's actually scary because (edit: for them,) high quality writing is harder than high quality coding, and it can be avoided quite a bit.
On the project side, it's rare for the implementation and verification stages to not consume all the budget and more, and delivery creeping past the original optimistic date. So there's no time or money to spend on documentation.
The combination is that even with your great advice in hand, it's hard to navigate to really solid and comprehensive design documentation underpinning the products.