As a former engineer at a YC startup from pre-A to post-B, I generally agree with much of this in a broad way if the startup is a technology first one with organic growth or hasn't really figured out product/market fit.

But I think some of the management and team stuff is much more complicated in B2B or B2B2C situations, regulated industries, or cases where there are substantial non-engineering employees, perhaps doing sales, onboarding, or things related to the "offline" world (if there are physical aspects to the business).

In particular, I don't think you can have a super flat eng structure run out of a few docs if eng needs to be working with one or more teams larger than the eng team itself unless there's some kind of separate interface to large outside teams.

If you end up with a significant sales team, account management team, support team, significant numbers of contractors, or other categories of workers because of the nature of the business, you will have to be more regimented about how things are structured. In companies that face this issue, it's often one of their major challenges and not avoidable compared to other kinds of startups - your sales team may have all kinds of ideas and some of them may even be good, and some may even want to sell them before you've built them. And if your sales team is 2x the size of product and engineering... it's not easy to run out of one document. (Note that I don't love or endorse this, but in certain kind of markets and products it seems like a bit of an unavoidable issue.)