See Windows 11 could run on Linux. That talks about the possibility of the Windows NT kernel being replaced by Linux. So it is not like the Windows Subsystem for Linux where Linux is on top of Windows, it is Windows on top of Linux.
Also see Windows Core OS: The complete guide | Windows Central. It says nothing about Linux and it says that there has been nothing official about the Windows Core OS but it does say it is a reality and we are likely to hear about it sometime this year. It is interesting to read the explanation of why Microsoft must replace the Windows NT kernel.
Now I might be the only one but I suspect that Windows Core OS will have Linux as its kernel. That is entirely my speculation.