Yes, according to the drivers
Yes, according to the drivers manual, your brief description of how the Driver Focus system works is correct. The manual does not mention how long it waits for you to be totally unresponsive or how it makes that determination. Having driven our 2019 Forester Touring for 3 months now, I am almost never allerted to stay focused because you have to have your eyes "off the road" for more than 3 seconds. That's a fairly long time to not be looking at the road. All-in-all, I find the new Eyesight and Driver Focus safety systems very useful and easy to live with. They are not overly intrusive and do not constantly go off unless you really are not paying attention or not driving within the road lines.