ffmpeg colormatrix operations only for 8bpc?
Yes, as it says a couple of lines above, colormatrix supports only 8bpc (8-bit per component) pixel formats. So, when a 16-bit input is filtered with colormatrix, it will be converted to 8-bits. And won't get converted back to 16-bit unless manually done or auto-converted due to output encoder limitations. But that's pointless anyway, as the precision has been lost.

Use the colorspace filter.

-vf colorspace=all=bt709:range=tv:format=yuv422p10:iall=bt601:irange=tv


Depending on your use case, format may need to be yuv444p10. Depending in your input, irange may need to be pc.