I searched thoroughly (google-fu isn't my strongest ability) but I cannot find any mention for it.
If you look at page 204 of EtDP, you notice that the last lore check for the abyssal giants mentions they are immune to slashing attacks. Strangely, there is no such immunity in their stats. Is this an error and, if so, which one is correct? I think that being immune to slashing damage at CR 11 is a bit too much, so I think the lore fact is the one that was written erroneously. However, given that they already wrote an immunity to charms, fire, acid and cold makes for a wonderful polymorph solution, albeit at a really high CL (17)....