Hi,
Although I'm not Festool, but I can answer your question: No, CXS batteries will not work with the SysRock.
It's a little confusing for someone who doesn't know about discontinued models/batteries, because the SysRock will actually work with "old style" 12V batteries that share the same interface as todays 18V. Thats why they put "works with 12V" in the description.
Kind regards,
Oliver