He seems to be confusing the Portsmouth service with a London suburban commuter route. Well it is a commuter route to some extent, but it isn't the sort of route Grayling was actually on about. No doubt RMT are well aware that the Aventra fleet for use on suburban routes, and the whole Windsor side service including Reading, will be standard class only, as that has been published by SWR.
But if the RMT view is that first class is wrong on journeys the length (and for the time taken) of the Portsmouth Direct, then there are far more possible targets out there...