The hidden service is a tor node itself and there are no exit nodes involved. As long as there is other traffic passing through the nodes involved an observer can't tell which traffic is for a HS. Maybe :)
Regardless, even if the hidden service is a node itself, a passive observer can still do traffic correlation attacks. It just requires more resources.
[0]https://www.torproject.org/docs/hidden-services.html.en [1]https://www.torproject.org/docs/tor-hidden-service.html.en
You are correct that a HS node doesn't have to be a bridge, etc. But it is a good idea to mask the traffic.