Your assessment is more or less correct, yes. The rule engine in a workflow system only needs to do a very limited number of things in response to rules firing -- create new workflows, update workflow state, and send email, more or less -- and the inputs are similarly limited, so they can often get by using only a very rudimentary rule engine.
I don't know of a commercial workflow engine that uses Jess, but then, I'm not always aware of all the things licensees are doing. We've had some discussions of Jess and workflow engines here on JavaRanch recently; try searching in
Java in General (Advanced.)