Well, yes ... playxvc.bat uses xvcdec to decode *.xvc to an intermediate format mpv recognizes immediately. I would assume Y4M. But if you have an xvc video stream multiplexed in an MP4 container, xvcdec won't recognize the xvc video content stream inside the MP4 container. You would have to demultiplex it from the MP4 first, then decode the raw xvc content, and then forward it to the player ... very complicated.
Instead, use the ffplay inside
ffmpeg_xvc20190121.zip provided by jonathans.