Must say I still find this confusing. At the very least, and as the Report describes, it won't offer anything like the additional movements of a third runway.
If the first part is being used for arrivals and an aircraft is on finals, will a departing aircraft be cleared to line up on the second part? If it lines up and holds it's vulnerable to an overrun (however unlikely) of the arriving aircraft and if it's cleared for take-off it's vulnerable to a late go-around by the arriving aircraft. And if the first part is being used for take-offs and the second part for landings, the departing aircraft will have to hold until the landing aircraft has touched down, if not actually cleared the runway, so possibly a small increase in movements there. Finally, if the extended runway is being used just for take-offs or just for arrivals, it's possible to envisage slightly closer separations but the overriding concern will always be to ensure safety if something goes pear-shaped for an arriving or departing aircraft and this will always restrict the total number of movements compared to an independent separate runway