I agree that this is contradictory. If you want to start a war, you don't attack from afar with cloaked, untraceable missiles. Unless Marcus' plan was to use all 72 torps to wipe out the surface of Kronos in one volley (i.e. knock em down before they even knew what hit them), it's just another example of bad writing.
No, I see the sense of it. Suppose we have this scenario:
1. Enterprise, at but not actually past the border, fires the 72 missiles.
2. The missiles cloak, and reach Kronos, doing widespread damage.
Plausible responses, as I see it:
1. The Klingons, not being idiots, would conclude the Enterprise attacked them and retaliate.
2. Admiral Marcus would insist that the Enterprise never entered Klingon space and that there's no evidence of anything launched from Enterprise toward Kronos except maybe nasty looks.
3. Federation public opinion would interpret the explosions on Kronos as being a Praxis-like natural disaster used by the Klingons as pretext to attack and destroy an innocent ship of the line.
Thus the Klingons are roused (fairly, I must say) to war, with an incident that will look to non-warhawks within the Federation as an unprovoked attack. It doesn't
guarantee war --- nothing does, if the heads of state are determined not to have one --- but it would be an awfully hard incident to get past.