The main use case, in my opinion, is for tests/CI. SQLite has traditionally been used to quickly run tests, however, if your actual infra uses PostgreSQL then the value is limited.
You think the main use for sqlite is running tests?
I think they meant sqlite is often used in CI/CD testing environments as an alternative to running a client/server database in these environments. For simple crud webapps, or frameworks that are db agnostic it works well.
My read is that the person you're responding to thinks that pglite could be a better fit than sqlite for ci/cd, where currently sqlite is used.