Having the battle last more than a few days was unrealistic in the first place; which is one of the resons why Abnett revised the timeline.
Sneak attacks don't get to last several weeks. In fact, if your enemy is still fighting you with most of his power intact past Day One, you clearly failed at a "sneak attack" and you're in all likelihood about to be annihilated.
Where was it said that the battle lasted more for a few days in the previous version? And why would that be unrealistic otherwise? It was indeed the entire point of the previous fluff that the Word Bearers tried to eliminate the Ultramarines but got bogged down in warfare.
Moreover, Abnett's portrayal of KNF is much more poignant than the old lore. Calth before was just another side battle in a long litany of HH side battles. KNF turns Calth into a true defining moment for the Ultramarines - it's their version of Pearl Harbor or 9/11, with the novel (which in-universe is described as an Combat Record) serving as their memorial for the fallen.
I wish I could only share your opinion. For Calth became utterly cheapened after Know No Fear and I cannot find anything to be proud about it as an Ultramarine fan, which is one of the primary reasons why I stopped playing them.