Still cphalcon/phalcon/session/adapter.zep
implements cphalcon/phalcon/session/adapterinterface.zep
so body of method should be the same across all adapters. It's justa bug, if you will try to extend Redis adapter in php code you will have most likely exception there.
redis.zep not implements but extends adapter.zep
so I do not think it's mandatory
cphalcon/phalcon/session/adapter.zep
must implements all abstract function cphalcon/phalcon/session/adapterinterface.zep
but not cphalcon/phalcon/session/adapter/redis.zep
With heritage this function just override the parent function, and can require other parameters
it's the difference between extends (herit) and implements (interfaces)