Your analysis is correct. Most POTS lines do not provide any indication that a not-yet-answered call has been abandoned by the caller, other than cessation of ringing. A standard US ring is two seconds on and four seconds off, i.e. rings occur every six seconds. So, at four seconds after the start of ringing, the OBi has no way of telling that the caller hung up.
If the POTS line has caller ID service, you could configure the OBi to not forward calls that originate from your other location. For example, in lieu of *72, set Line Port -> InboundCallRoute to
{12123456789:ph},{ph,pp1(ob234567890)}
where 12123456789 is your GV number (in the format that shows in Call History) and 234567890 is your OBi number. Then, your calls would ring only the Phone port, while calls from others would ring at both locations.
If you don't have POTS caller ID, IMO your only options are to either delay the forwarding to e.g. 10 seconds, or live with the problem. Perhaps RonR will chime in with a better solution.