You are right in the point you are making. We can do no more than assume for the dev.No, it is assumed that it is read. It has only been shown that account created by the developer logs in at regular intervals (and has done so for the 6 years the account has existed - but almost never interacts in any thread). There is no listing of activity ("Last Seen: XXX minutes/days/whatever ago viewing XYZ thread"). It may be a slight difference - but it is a very distinct one.
I was actually talking about Patron subscribers, a few have commented here in the past, with them its more than an assumption