In theory it should be possible to use the Linux kernel and create an OS that doesn't resemble Linux distros like Ubuntu. You could strip out systemd and put in a custom init system that doesn't conform to the typical Unix daemon startup expectations. You could write your own X11/wayland-like display server thing (like Android does). You could replace BASH with some other terminal (even something like PowerShell).

Some things are baked into the kernel and are tricky to work around. Like the execute bit. If you wanted to make a Linux distro that can run arbitrary binaries downloaded from the internet (like Windows) you'll have to contend with the kernel refusing to execute a binary that doesn't have the x bit set at the filesystem level. (Can you even execute files from fat32 or NTFS? Hmmm...)

Another example is the Unix file system layout. If you wanted to show the user a DOS-style drive-based layout (I.E. no /mnt or /bin or /etc, but instead C:\ and D:\) then you have to fork GTK and GNOME to know how to obfuscate the file system like that.