All the little chicks with crimson lips, go...
Hey, get your stinking cursor off my face! I got nukes, you know.
That's certainly what's intended. Rule 13 is clear and similar to the Siegfried line and German Cities in Liberty Roads. It makes the defense of the 13 advanced fortifications more attritional. Even at 6:1 there's only a 10/36 chance of a defender tactical results of D3 reduced to s. If the German has enough reinforcements to pump in he can, even if the hex is isolated. Replacements can't though if there's an adjacent russkis. German cities don't benefit nearly as much as even D results in an S. My advice to the russians is get stuck in and push on around the fortifications to other objectives.
I agree with Colin's interpretation of the rule. Those places are hard to take, if the Germans are willing to keep funneling in loss-takers.
And who said there was nobody on BGG that knows this game?