Jenkins.doQuietDown(boolean block, int timeout) blocks as long as there are jobs running. This is not what it says in the documentation and a less useful behavior in my point of view.

boolean block
usage="Block until the system really quiets down and no builds are running"
int timeout
usage="If non-zero, only block up to the specified number of milliseconds"

I suggest the implementation should do what it says in the doc, i.e. only wait timeout, then return even though jobs are still running.

This error should be easily exposed by a unit test...