Yes, a Timing step's duration includes its children Timings' durations.
It's easier to visualize as a tree*:
> http://localhost:8203/ExternalService.svc 73.4 ms
>> GetOrders Service Requests 56.1 ms
>>> _externalRepository.GetOrders 55.9 ms
So the root timing (i.e. the entire request) took 73.4 ms; GetOrders Service Requests
was 56.1 ms of that entire 73.4 ms; and _externalRepository.GetOrders
was 55.9 ms of its parent's 56.1 ms.
* the MiniProfiler extension method RenderPlainText()
can output this format.