I dont know know where people are getting this idea from. Peter is very clearly talking about the foregoing parable Jesus had just concluded about readiness. Jesus answers peters question about the parable by asking him a question that only he and the disciples knew the answer to, "who really is the faithful and wise steward?"
peter and the other disciples witnessed Jesus give Peter the Keys of the Kingdom in Matt 16:19 thus appointing him as the house servant (faithful slave). The steward was a cultural custom of the Jews going back over a thousand years. The appointment of this steward was always demonstrated by the master giving him his keys (compare Isa 22:20-22, where the exact same verbiage is used in the exact same event).
The Watchtower is completely ignorant of this cultural custom, or if they aren't they choose to ignore it because recognizing it eliminates their claim to the title.
but my purpose in elaborating like this is to show that the faithful steward is absolutely NOT a parable. It was a real custom back then, dating back ages.