That was before Kishi even introduced Kekkei Genkais. How would Hiruzen perform Sharingan techniques or the Hyuuga's Byakugan or Jyuuken?
Or unique techniques like the Aburame's bug techniques when he doesn't have them in his body?
Or Water, Lightning, and Wind techniques if he doesn't have all the elements?
I almost never go against Kishi's word but this is an exception.
This wasn't before kishi introduced kekke genkai's..... Do you not recall haku? And look at what itachi said carefully, He could use all existing Konoha jutsu, Firstly I hope you know sharingan isn't a jutsu, Its a doujutsu, a genetically passed down bloodline trait.......Same with byakugan....The thing sharingan uses- aka tsukyomi etc are the jutsu, In anyways, its obvious when hiruzen was appointed "Professor" which would be sometime in his prime, back in the 2nd hokage era, Noone had any mangekyou sharingan abilities....The last known users were dead or supposed dad (Madara/izuna) Konoha was just founded and couldn't have been as big as it is now..... At first there were only 2 clans (Uchiha and senju), others joined in afterwards, Wind has always been a rare element in konoha, so i doubt he has that, but its very possible he has the other 4....Orochimaru his own student, Iruka and that special jounin guy seem to all believe he can use a shit load of jutsu......the special jounin guy told konohmaru over 1000 techniques , which is comparable to that of kakashi which he copied, You just need to note Orochimaru said it in PAST tense , he knew and could.....Not what current translations say, know and can use, And ps - Hiden techs like shikimaru's ect be taught to anyone, they just keep it in their clans
Its just a matter of kishi clarrifying what clans were in Konoha at the time he was appointed professor
What makes me truly believe this, Is that Minato was supposed to be the only one able to use the Death God Reaper seal, and he learnt it, It seems he just has a talent for learning people's jutsu lol