So here's my take:
We have two datapoints:
1) In Turn Coat, Harry makes a certain connection between Rashid's mention of a grudge and the limp that Demonreach appears to have.
2) WoJ that the limp was caused by a glacier and Rashid "focused the tank" (which I personally take to mean that Rashid and DR were on the same side).
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First thing is, how could a glacier affect Demonreach?
a) Either Demonreach was created before the glacier appeared -- kinda hard since the last time the glaciers retreated was ~20,000 years ago. Granted the original Merlin did some timey-wimey stuff during creation, but still, Merlin himself was around ~1500 years ago, so why build Demonreach already wounded?
b) The second (and more likely IMHO) is that Demonreach got wounded with a glacier *after* it was created. How did this happen? Again, timey-wimey stuff. We've seen in Cold Days that the way to attack Demonreach is through some time-related magic. Obviously there was a previous assault. Consider the following notion: The bad guys cast a spell to divert a glacier 20,000 years ago so that it will overrun the foundations of Demonreach, retroactively weakening/destroying him.
For bonus points, take a look at the Amber Chronicles (either book 9 or 10) where someone tries to attack Corwin's Pattern in a similar manner. Jim's extensive connection with the Amberverse might be a clue here.
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So if Demonreach was being attacked with time-related magic, maybe Rashid didn't make his top priority stopping the spell -- attacking things in "real time" instead. This ultimately worked (Demonreach is still standing), but Demonreach did get partially injured, which is why it still holds a grudge.
Final note is that Rashid might have well known that he was doing thing inefficiently, but had different priorities. As an example, if innocent lives were at stake, Rashid might have preferred to prioritize saving them, knowing that Demonreach can take a beating.