I might be wrong, but it was nowhere implied, that Hiruzen can use all the techniques of Konoha but simply 'knows' them. The scrolls behind Hiruzen are a collection of all known Jutsu (at least of Konoha) for the only purpose of knowledge, any Hokage has access to. Konohas arsenal of Jutsu consists out of a bunch of varied techniques, started with the 5 basic elements, up to hidden Jutsu, Kekkei Genkai and Doujutsus, such as medical techniques and Kinjutsu. Orochimaru himself claimed, that it's impossible to learn all Jutsu in one life, making it impossible for Hiruzen to learn any technique of Konoha whose performances were documented in scrolls over generations, especially if we consider that Orochimarus life consisted out of studying any type of Jutsu in the long run while Hiruzens 'meaning of life' was defined differently.
There are a plenty of techniques Konoha has to offer from various sources:
- The five basic elements: Katon, Raiton, Doton, Fuuton, Suiton
- Hidden Jutsu: The techniques of the respective clans
- Kekkei Genkai: For example the Mokuton
- Doujutsu: The Kyuugas Byakugan and the Sharingan of the Uchiha back then
- Medical techniques
- Sealing techniques
- A plenty of other abilities, based on Nin- Tai- and Genjutsu
I'm sure, that the scrolls piling up in the Hokages room document all the techniques the village had and has to offer and is getting updated or extended whenever there appear new. Even if we take a look on the element Katon Hiruzen showed in performance, yet you could ask yourself whether Hiruzen could really use all Katon techniques the villagers knew over generations, to take an extreme example like Uchiha Madara, as a talented Uchiha whose clan was known for using powerful Katon techniques, the scale of Madaras techniques were on such a massive scale Hiruzens stamina couldn't even bear.
So this actually is a contradiction, on the one hand you say, that the scrolls document every Jutsu Konohas villagers were using up to now, yet it's just impossible for him to learn all of them, not only in consideration of the time it would need but also due to his predisposition. The mastery of all 5 elements itself would take Hiruzen already a large amount of time, let alone to learn all respective elementary techniques. Even excluding the Hidden Jutsu, Kekkei Genkai and Doujutus which were surely documented in the scrolls and in fact used by Konohas villagers, there would still be a too large bunch of techniques and abilities he would have to learn, apart of all 5 elements, medical techniques, all Tai- and Genjutsu techniques (the eight gates included, which were supposed to make the respective being to
overwhelm even the Hokage) and the clans personal abilities (the Senju-clan alone was supposed to have members with thousands of Jutsu, such as the Uchiha-clan was supposed to be an extraordinary talented clan).
As talented as Hiruzen might be, no one would master such an amount of techniques in his life-time, considering his bloodline which doesn't even allow him to learn certain techniques and his stamina, which couldn't bear them.
In my opinion, it was more hype than actual feats done by Kishimoto, who took Hiruzen into the right light at the beginning of the series and made him out to be the strongest Hokage, while even Hashirama was made out to be inferior to Hiruzen in his old age while being in his best state and immortal (Hiruzens statement claiming to take out both the first and second Hokage for sure in his old state, just proves Kishimotos invalid given reputation).
Hiruzen surely
knew every technique villagers of Konoha used, but he surely can't perform all of them, this is merely hype given him by Kishimoto as he is the Hokage who was supposed to have surpassed all Hokage. But actually, he's trash and his existence didn't have a meaning in Naruto Shippuuden, though one could say, that the 'god of Shinobi', known for the mastery of all arts and techniques of Konoha and the one who surpassed Hashirama, could have been kept in a far better light instead of being left in oblivion and put in the shadows. In Shippuuden, Minato however replaced Hiruzen to a certain degree when being supposed to be Konohas greatest Shinobi, who stopped the war, developed several high ranked techniques and knew sealing techniques.
