Dimi, a few misconceptions from your side: The 737 is fail passive, well, that is partly true, but wrong as well. The NG allows fail operational operation since 2001, you just have to buy it (no idea if THY did that). And true, if the RA1 fails you should not get the RETARD mode, but it seems from the preliminary report that it didn't fail but merely reported the wrong height. A fail i normally shown with a flag and not any height indication unlike in this case where it did show something.
And of course, as has been pointed out several times now, autothrottle RETARD is independend of any autopilot mode and in fact a single channel operation at all times depending on RA1.