GrapheneOS and AOSP are Linux distributions too. Linux isn't glibc, systemd, Bash, coreutils and Wayland. Many Linux distributions use a different libc, init, shell, command-line utilities and graphics layer if they even have one at all. Android having massive adoption and high usability doesn't mean it isn't Linux. Linux dominates mobile already.
SailfishOS doesn't use a typical desktop Linux software stack anyway but rather their own largely closed source UI and application layer.