If the twin theory is correct (I don't believe it because there's no reason whatsoever for 'Ciel' to pretend to be his twin - the family is dead, it doesn't matter who he is), then I agree with that being the lie. But Sebastian doesn't lie, so... I doubt the twin could have been/was reanimated - Soma would have questioned why Ciel was doing what he was doing as he was running away... I also think that Agni could have closed himself in the cupboard with Soma - the door opens in, he could have used his weight to hold it closed.
I won't ague the Twin theory with you as there are arguments for both sides and we don't have enough evidence to disprove the theory.
However, I will say Agni would have died regardless. Doing things the way he did Agni guaranteed Soma would at least survive the encounter. Sine we were dealing with attacker/s with guns - and even 19th century pistols could penetrate a wooden door and still have enough force to kill a person the other side. The attacker/s could have - and would have - easily shot Agni through the door and we still would have lost Agni - and possibly Soma.
Further the attacker/s had a mission - to vandalize Ciel's room and leave before he returned and eliminate all threats to the success of that goal. So any smart attackers wouldn't leave any threats to their mission free to roam around nor would they expend valuable time trying to go after a person who was not a target or a threat to the mission.
By holding the door shut from the outside, Agni gave the attacker/s the free access to the biggest threat at that moment, while simultaneously eliminating Soma from still being a threat by locking him in the room. So unless the attackers are revealed later to be utterly sadistic sociopaths - thus leaving Soma alive would likely be OOC behavior - going after a young man locked in a room would have just been a waste of time and resources, since Ciel and Sebastian could have returned at any moment.
Could it be that if the twin theory is correct, and presumably the lie Ciel is referring to is everyone was dead when Ciel and Sebastian formed their contract...
Would it make sense that the other twin was some how reanimated by Undertaker, and then ended up like the teacher in the School arc? As it was shown that particular corpse seemed rather realistic and was able to out run Sebastian going at full strength.
If this is the case, it could explain how someone could take down Agni and still fulfill the requirements for Soma actions and the various reactions Soma and Agni had to the current senario.