Hermes is not yet available in the managed workflow. See the feature request.
What you can do in the managed workflow is the last paragraph on the page you linked to:
You can analyze the contents of your JavaScript bundles using react-native-bundle-visualizer. It will show you how much each package in your project contributes to the overall bundle size.
You can improve startup time on low-end Android devices in the bare workflow by opting in to using Hermes and precompiling your app to Hermes bytecode at buildtime.