One of the reasons is that pax won't be expected to walk from one end of the train to the other. Hs2 stations will be designed with multiple entrances/exits from the platforms so people will be able to get on/off far more quickly.
The number of entrances/exits from the platforms influences the ease of access to the platforms, not the ability to get on/off the train more quickly. The latter is determined by the number of doors on the train and door widths/layout etc.
If you observe such as the German ICE services, their dwell times are no better than existing UK trains.
Any suggestion that HS2 services will enjoy improved dwell times is like much of the projects supposed benefits-wishful thinking