It is also worth noting that Zahariel is one of three characters in the Horus Heresy confirmed to have both contact with and assistance from the Watchers in the Dark (the others being Primarch Lion El'Johnson & Captain Garviel Loken of the Luna Wolves). Due to the aid of the Watchers in the Dark, Zahariel's power-set accounts for Cypher's tabletop rules, such as his ability to slow time Matrix-style (BS:10, I:8) and his ability to phase through objects (so can escape from pretty much anywhere you lock him up).
Of course Zahariel is a Librarian and Cypher's tabletop rules do not have the Pysker rule. However if we view Cypher like the Eldar who are all pyschic, the Pysker rule only applies to those Eldar who use the warp to cast spells. Also the Watchers in the Dark have demonstrated the same powers as Cypher when breaking Loken out of jail.