The changes made by the Vox Populi team are much more extensive than that.
When the source code for the DLL was released numerous changes to AI functions were closely examined and fixed, even new ones added, so that the core mechanics of Civ V, as played in Vox Populi, are much more than what was released by Firaxis.
I haven't yet seen the code for Civ VI but I will put good money on there being similar, if not identical, AI routines included in it. This is what I am talking about. The core code that defines the AI is in the DLL and a small team over a period of 2 years has produced a much better AI than a fully funded software company has in 5 years with many more coders in their employ.
Assuming they are still coding in C++, and even if they're not, examining the code changes would provide them with all they need even if they didn't take the repo and merge it into their own source. Coding for a game may not be easy but it isn't rocket science either, anyone with a modicum of coding can see what needs to be done and do it, the fact Firaxis hasn't tells me they don't value a working AI too highly.
I didn't mean to diminish the efforts of modders, but modders' efforts is still what they are. It wasn't something that was built from scratch. Examined! Sure. Learned from? Undoubtably. Improved on? Without
question.
I have experience in both modding, developing, and iterating on what developers have done myself. Not in C++ (thank the heavens

), a lot of other developers rely on a more virtualised layer (LUA is a common choice) as supposed to the hardcoded constraints of binary files. I get where you're coming from. But you're bigging up Vox Populi while dressing down the developers.
You're making assumption after assumption (that the AI routines in a game with a fundamentally-different engine base are "similar", for the latest one) while dressing down the effort required for games development programming. It doesn't have to be rocket science to be difficult. A working AI is
difficult. Do you know how long it took the contributors to Vox Populi to build the AI? To tweak it? To iterate on it? These are the resources they have that Firaxis likely don't - games development is done on incredibly tight deadlines.
And while you may pan this as "not valuing a working AI", a) that's incorrect because the AI
functions, it just doesn't function to peoples' likings, and b) it's irrelevant what the developers "value" when game mechanics, render tech and pretty much everything else will take precedence. And unlike larger pieces of content, an AI can more easily be improved, post-release. As can text fixes, UI polish, etc. That's why these things slide down the "priority" list so often. I'm not saying it's ideal, but that's the industry as it stands.