I'm assuming that the rest of the campaign books won't be featuring the main rules
That's correct.
Down the road the plan is to release the core rules as a separate reference so we won't need to bring our Kursk book along to every game. At that point, someone new to the system will need to buy the core rules (I'm betting on a soft-cover) along with whichever non-Kursk theatre book they are interested in.
I can understand how someone could feel frustrated if what they have heard about the rules piques their interest, yet the idea of wargaming Kursk does absolutely nothing for them. I get that all the time at our local club when friends present their favourite rule-sets for periods I find dreadful.
However, a good game really can do wonders for a subject or a period that previously seemed dull.