That doesn't mean it wasn't in Nadd's holocron though.
Nadd's holocron could've been lost when Gravid did his thing.
Sidious could've found Sadow's holocron when Plagueis was still keeping stuff from him.
Sidious may have had Nadd's holocron, but not looked into Sadow's alchemy before finding Sadow's holocron. (Book of Sith does confirm he wasn't too invested in Sith alchemy at first).
Or Sidious may have already had Sadow's knowledge of alchemy before finding Sadow's holocron, and the holocron thing was simply an anecdote.
None of these possibilities preclude the knowledge being in Nadd's holocron.
Regardless, at that point in time the Sith had at least the alchemical knowledge of Naga Sadow and the alchemical knowledge of Sorzus Syn.