Although the example follows on from the previous example (where Kris has just been engaged by Ungoliant's spawn and forest spider), i think each example is to be taken in its own right.
In the actual combat example, it doesn't state anywhere that the forest spider has just engaged Kris during the previous engagement check and therefore it shouldn't be assumed that that is the case.
The fact that the attack is undefended is irrelevant; the forest spiders +1 is only appleid in the combat round after the forest spider engages, and the example is simply of a later round of combat, which has assumed the forest spider didn't engage in the previous engagement check.
If the forest spider had engaged in the previous engagement check then the +1 would be applied regardless of whether the attack was defended against or not.
Hope this helps