That isn't to avoid churn, that is to have more people around in general. If you leave your old team it means you no longer use your knowledge, the important part is to have engineers around, not to have the same engineers working on the same tasks.
For example, Google encourages churn by making it really easy to move to other teams in other areas. You don't talk to your old team again, so that is effectively the same thing as you leaving the company and them getting a new engineer, ie for the teams it is equivalent to churn.
