Talzin only partly completed her drain ritual, and according to Viscus the process would have left her weakened anyway. Also, either way, on Dathomir she was no more amped than Sidious.
Either both were amped by the Dathomir nexus in a Legends-esque manner (Legends is where Talzin needing to be around plantlife and dark side nexuses are elaborated on), or from a canon PoV neither were, since nor the plantlife nor nexuses have been expounded on.