Geoff doesn't say anything about tapping in or out pre/post 1600. The implication is that in reality peak fares don't apply until 1605, with the further (but unclarified) implication that you can tap in after 1600 and catch a train that comes almost immediately and still be outside peak hours, rather than tap in before 1600 and then 5 minutes kicks in after 4pm.
When you are using contactless or Oyster PAYG, the words "peak" and "off-peak" apply to the time you pass through ticket barriers or validate your Oyster to begin a journey. They do not apply to the time you travel nor do they apply to trains.
If you tap in before 0635 on a working day, you will pay the off-peak fare even if you sit around in the station until 0700 before travelling, provided that you do not exceed the time limits for your journey. I am sure that there are some people who do this in big London stations that have cafes inside the barriers: you could get a "free" breakfast with the savings as long as you leave your house in time, even if you don't actually need to leave so early.
If you lived very near to an unbarriered station, you could even go to the station and tap in before 0635 then go back home for 20 minutes. You would pay the off-peak fare even though most people on your train would be paying the peak fare. The train companies would not be too happy with that if they caught you but I am not sure if it is actually against any rules.
If you use a national rail ticket (orange card, e-ticket, etc.) then peak and off-peak apply to either the scheduled time of a train, or the actual time of a train in some cases. They do not apply to the time you pass through ticket barriers even if ticket barriers are programmed to reduce abuse.