This is the most logical and likely explanation, IMO.
It's not really that he knows the word so much as how and when he used it. "Damnation" and "damned", sure. But using "damn" as a curse in that scene as opposed to a word specific to Roshar or to the Singers themselves seemed strange, out of place and noteworthy to me at the time. (Ninja edit: After all, the Rosharans used "Damnation" and perhaps "damned" but have you ever heard them say "damn" rather than "storming"?)
I don't know if other characters have used the "modern earthling idiomatic English" (as @AquaRegia put it) of curse words, but I couldn't specifically recall any, and for that word to appear in that way stood out to me. Especially at the very end of the fourth book, after how many thousands of pages of not being used? Almost like Brandon was using it to draw attention to something.
And, if that is the case, and Brandon is drawing attention, I was pondering what the reason would be. Maybe it's just to show that the Fused are from an ancient time when that word may have been used that way (i.e. before the Desolations), or perhaps to hint at their off-world origins.
Just my thoughts. *shrug*