My explanation above just gives the illusion of entire computer encryption.

Say you have a separate hd for each OS. Each with bootloaders on their drives. To bypass grub running luksopen you can boot directly into windows in the bios, in this instance the windows bootloader will be used to load windows. However if your bios is set to boot your Linux HD and grub has successfully found your windows drive and created a boot entry for it, it should be selectable after luks decryption. This can give the illusion that windows is encrypted while not really being so to an advanced user. There is nothing preventing you from mounting windows as its not really encrypted, just the way grub loads Luks before OS selecton. If I remember correctly systemd-boot loads OS selection before luksopen giving no appearance of encryption till after your OS selection, you should be able to boot windows without the false sense of drive decryption.