There's about three misleading things here. Firstly, replacing "could" with "will" is just plain dishonest. No wonder it looks a stupid argument if you make it one.
Even "could" is a bit overblown, and Sky (and probably all media who report this) shouldn't have used it, although I suppose it's a bit snappier than the headline: "'The implied commonality of spirit provided by the EU has played a role in helping to maintain peace on this continent (at least among its member nations), a role that is put at risk if the UK leaves the EU since this could easily lead to the break-up of the EU; while this hardly guarantees that war will follow, anything that provokes or exposes disunity is hardly likely to improve a situation that could well be more fragile, in some areas, than people realise', says PM".
Cameron really shouldn't have been invoking the various historical dates he rattled off either. It sensationalises an argument that really shouldn't need it. He could just have stuck to "what happens in our neighbourhood matters to Britain... let's not pretend we can be immune from [European, and EU, affairs].", and left it at that.