KGT lists {nuH} as "possibility", and I think this must be an error. The
main body of KGT explains that {nuH} can be used metaphorically for
"possiblity", but the regular word for "possibility" is {DuH}.
(KGT p. 109+110)

What is the common treatment of this? Is there any information I'm
missing? Does {nuH }really mean possiblity, or is only used
metaphorically, like slang words are? In that case, the list should have
had a remark like other slang words.

It's not slang, it's a metaphor. nuH means possibility in the same way that Soj means subject matter. It's an actual meaning the word has picked up that can be used outside of slang contexts. The word literally means weapon, but you can use it idiomatically to mean possibility. We're only told that it appears in the idiom Hoch nuH qel consider every possibility, but the fact that it appears separately on the word list suggests that it can be used with this metaphorical sense wherever you like.

-- 
SuStel
http://trimboli.name