Premature and/or misplaced optimization. It’s kind of funny that they worried so much about std::string’s performance that they rolled their own, yet have this big honking lua layer thunked on top of everything for game logic. Wow!
We did a ton of tracing and perf/memory captures to identify that string allocations were a significant drain in many locations in the code. We don't see those issues with our other uses of std:: (vector, unordered_map, set, etc.), just with std::string. So it was a logical place to do targeted optimization.
We did that optimization before lua because of the fact that there's a very clean way to make the foundational debt into MacGyver debt, since there's a trivial conversion between std::string and AString. Sadly we haven't been able to come up with any bite-sized moves that we can do to phase out the wasteful use of lua as kvp storage buckets. It's an all-or-nothing problem that makes it a much bigger chunk of work to undertake.
We were in a similar situation and using our own string implementation improved performance and reduced memory fragmentation.
Some people refuses to accept that certain software cannot rely on general purpose libraries and need to roll their own solution adapted to their specific needs.
Usually standard lib vs custom arguments end up in the weeds like tabs vs spaces at game companies but ultimately it has almost nothing to do with framerate or runtime. Largely it is about that EGO. Why maintain a standard lib instead of improving gameplay and networking? Well some want to be a lord in their feifdom where they are the controller the code and the ring.
Riot Games has std::string and AString, but what happens when player two enters the game and you got BString? Then BString invites its friends and you got CString and DString. Now your 'standard' has many standards and is more standardy and like warring lords within internal factions like a Game of Thrones.