Qt Creator builds CMake projects by running cmake . --build, which then runs run whatever is needed based on how the project was configured: make, mingw32-make, nmake, or ninja, for example.
What it doesn't say is that to actually use Ninja, you need to:
- Force the kit to use the Ninja generator.
- Install Ninja. This one might be obvious to people familiar with the process, but I assumed it somehow would already be present if CMake was present.