You're half right... although the new Fritz 15 is a little stronger than Deep Rybka 4.1. Rybka engines were written by IM Vasik Rajlich, who was later employed by ChessBase GmbH to produce the latest Fritz engine.
F15 has many of the same features that used to be unique to Rybka engines, which are of no use to most users, but can be very handy when paired with the right software
What is the difference in Fritz 15 And Deep Rybka...?
from my observation Fritz 15 is only the other name of Rybka. They change only the name of existing engine and give to us....!!