This is correct. During the first release into a ROS distro, bloom will ask you interactively at the very end if you want to enable pull request testing. Also, you have to give ros-pull-request-builder access to your repo (or set up a webhook), as described here: http://wiki.ros.org/buildfarm/Pull%20request%20testing .
If you have already released a repo into a ROS distro, bloom won’t ask you again (even if you run bloom --edit). Then you have to make a manual pull request to rosdistro where you add the test_pull_requests flag).