V, disguised as Rookwood, connects the dots for chief inspector Finch & inspector Dominic in the abandoned tube under London.
previously to that meeting we learned:
"your records are either deleted omitted or missing"
"the original electronic records have all been lost"
"patient V could not remember who he was or where he was from"
Chief Inspector Finch has already figured out that someone in the "gov" released the bio-weapon on their own population, when V tells him in the tube exactly who's idea that was - Mr. Creedy.
Considering all the records lost and V's own memory loss, how could V possibly have known that specific detail?