Turns out he know konoha style ninjutsus, like you know kirigura jutsu have water prison, mist, and water dragon as their ninjutsu? Well for Hiruzen he’s the only one that knows all of Konoha’s type of techniques. The proctor during the chuunin exams before he died by the sand shinobi, he used one of the konoha style jutsus. Hiruzen knows 1,000 illusionary techniques as well.
The hiden techniques on the other hand are secret arts that Hiruzen does not know such as expansion jutsu etc. The only thing he can use from Choji’s clan is their physical jutsu which is “Human Juggurnat” in which Chinju in part 1 stated to be a konoha style jutsu. So Hiruzen can’t expand like choji’s clan but he can use human juggurnat.
I have a thread talking about this I can send you the link