This seems consistent with OP. You had a feature where most of his Gantt chart is, in effect, already done: you had a clear problem with a clear well thought out design/solution (with associated documentation) in mind, you had a well setup analytics process for deployment and followup... you really had everything except that big fat chunk in the middle labeled 'coding'. So in your anecdote, an agentic coding LLM really could deliver a huge speedup by doing the remaining 10% or whatever of the work.

This is why LLMs are really great 'knocking off the todo/wishlist' of things you always meant to do. The problem, as far as broader discussions of 'productivity multipliers' or 'total factor productivity' go is that there's a certain perverse diminishing returns to such wishlist items (if each item was all that important, why didn't it get done before?), they generally only apply to a small part of a large complicated whole (what % of your ecosystem/business/community as a whole is the login page, as pleasing and profitable as that fix is relative to the investment? Probably not a big %), and they are also finite (what happens when you have worked through your backlog of lowhanging fruit?).