I think we are splitting hairs here, but it could come down to the last minute or two so it's important to get this right.
Your interpretation would not work because a completely arbitrary time would be used when the first passenger was allowed to leave the aircraft which would have to be recorded somehow. No accurate time of release would be available.
Using the 1st door open method gives an automatically recorded time and relates to the moment the passengers are 'released' to resume their business.That could still take some time in real life situations just like passport control, baggage etc