-I think it meant he knew of all the jutsus and probably knew there strengths and weakness. Also Hiruzen only had like 2 chakra natures right, so he couldn't know everything.
Thats what i believe they meant also by saying he knew all of konohas jutsu. He might have had more natures, but didn't display it during the fight. He probably only used what he needed. I also find it highly unlikely for him to know ftg.