CodeBuild with parallel builds

Question

I'm using CodeBuild with this syntax in buildspec:

version: 0.2
phases:
  pre_build:
    commands:
      - pwd
  build:
    commands:
    - echo "hi" && sleep 50
    - echo "hi" && sleep 20
    batch:
      fast-fail: false
      build-list:
        - identifier: build1
        - identifier: build2

It should be running in parallel but run sequentially.

What have I missed?

Try an example above