Algalon was capable of inter dimensional travel, deep space travel, having seen "entire solar systems gone in the time it takes for your mortal hearts to beat once" and personally has seen worlds bathed in the maker's flames (as in, he has seen the Titans in action before their deaths, so he could be eons old for all we know). It sounds very, very far fetched that he wouldn't know they were killed.
I'm going to go with retcon.
Edit: I personally don't think the Titans are dead. Defeated and maybe incapable of reforming, but not dead. I think we'll see their "resurrection" someday in Warcraft lore.