This "postflight" term is way confusing as to what the meaning is. Duty time is report to release for a line holder and beginning of phone availability to release (if assigned a trip) for reserve. You are making it sound like a literal post flight walk around in your description and that is not the intent. You get a "debrief" time after block-in to allow for parking, termination checklists and removal of your items from the airplane while passengers deplane. Most companies I know of have 15 minutes of "debrief" time because you are certainly not OFF duty the moment you block in unless you are sitting on the plane as a deadheader.

So, the pilot would not be legal to go with a 0600 phone availability start and 2146 arrival + 15 minute debrief = 2201. On duty more than 16 hours (16:01) which equates to not being able to look back and find 8 hours of rest (7:59).

Now remember that is just for assignments. The devil is in the details of the paperwork, flight time, and the calculations of crew critical off time (CCO). Once you are at the runway, duty start + flight plan time + taxi in + debrief all combined have to keep you under 16 hours.

Also, Whitlow does not apply to international flying, though most regionals and some other companies agree to treat Canada and Mexico as domestic operations for ease of work rules and duty times (which is actually a plus because Whitlow is more restrictive).