Earlier quoted context omitted.
There have been documented cases of infections happening in seconds. Yes, longer exposure time increases infection probability, but the 15 min threshold is arbitrary, like the 2 meter social distancing. A bit like the "you can pick up food from the floor if it sat there for less than 5 seconds" meme.
How are infections measured in seconds?
Source: https://www.abc.net.au/news/2021-06-22/covid19-cctv-footage-...