Currently, when trying to build and run the example app, you need to navigate to packages/react-native-node-api-modules and run:

npm run copy-node-api-headers
npm run build-weak-node-api
npm run generate-weak-node-api-injector

Unfortunately the second step (build-weak-node-api) requires Ninja to build the weak-node-api. I would suggest to make the CLI check if Ninja is present and if not, then silently fallback to Makefile. It don't think that Ninja should be a required dependency, but optional. It only makes the build process faster.