From Volume 1:
"The airport would provide almost immediate relief to the pressing situation that is causing £2bn in potential trade to be lost to the South East each year we remain without additional runway capacity"
This implies that airlines that can't get into Heathrow, Gatwick or other London airports due to lack of runway capacity would switch to Manston in a heartbeat if it were re-opened.
The reality is that Manston was available previously, and airlines simply ignored it. It's been proven to be an entirely false assumption that airlines will happily use any airport as long as it's in the South East. If they can't get into LHR, LGW or to a lesser extent STN or LTN then they probably won't bother.