Heh...that entire intro, while cool and shocking (starting off with the explosion and foreboding interstellar jeopardy) was rife with errors of that kind. Although, to be completely fair, your supposition may have a glimmer of merit, as IIRC the Excelsior was fitted with a cloak-like "Stealth Screen" in the 80's-vintage DC comic book series after Kirk & co. acquired it post-TSFS. It was a really long time ago and I can't remember details, just a vague recollection. Or maybe I got it mixed up with some FASA RPG storyline. Honestly can't recall clearly.
^ I always thought that maybe Qo'Nos had planetary shields that protected them to a point.
I could buy that - but, as you say, to a point. Say that the planet itself could have been largely protected - even shields would overload after such a withering force at that close a range. Additionally, non-shielded nearby worlds in that system would likely have been pulverized, or their orbits shifted as to do far more long-term geologic/gravimetric damage to Kronos than just a dodgy atmosphere. That aside, most of their entire fleet, space stations and drydocks in that and neighboring systems (not within the theoretical planetary shield), including Kronos 1 would have been decimated in the first nanosecond of the explosion - if, in fact, it was traveling so fast as to hit the Excelsior within a matter of minutes. The shock-wave would have to have basically been traveling at high warp to get there and not a single corporeal being in the vicinity would have had the time to turn on their shields before the shock-wave hit them. Sulu's crew barely had enough time to react themselves before the wave front reached their location.
If the true measure of destruction culminating from this explosion was explored, it could be argued that the Klingons never would have been in the position to wage any kind of war, much less border skirmish, after such a disaster. They would have been permanently crippled and never would have been able to negotiate at the Khitomer accords from a position of strength.
All in all, not an entirely unprecedented occurrence in the history of TOS Trek when things need to move at the Speed of Plot, but still pretty flimsy nonetheless without an unhealthy suspension of disbelief.
That being said, it's still one of my favorite TOS movies!
