Not a game dev in any sense but TFT uses the same engine, models, etc as base league, that’s why it’s considered a game mode. These other games are essentially being built from the ground up.
You nailed it. TFT basically uses ALL of the same infrastructure as League. That means you don't have to rebuild tons of stuff from scratch, and the humans working with the tools are already extremely comfortable with them. There are some downsides to the approach we used for TFT, but the upside is vastly higher dev speed (at least to start with).