devtrackers.gg
TFT isn't a fully rewritten game engine. It's the same C++ engine with a different load out of assets and scripts as SR, and likely a ton of those assets are getting reused. In fact most of the custom game modes that differ dramatically from SR (Nexus Blitz, Odyssey, Star Guardian) are using a similar strategy. The most obvious evidence is that they run the same executable.
In this engineering blog, they've stated that League is about half scripts (animations, interactions, champion skills, stat management, menu functionality) and half C++ (shaders, models, sprites, particle effects). Swap out the scripts, keep the same engine, and you can launch new game modes pretty quickly (maybe two or three times a year, like their current schedule).
The engineering feat I'm interested to see is how they got the C++ engine to run on non-x86 platforms, I'd love to see a dev blog written about what compilation strategy they're using to have the engine run on ARM based processors.