GrapheneOS (like any other AOSP fork) is technically a Linux based OS. They run a modified version of the Linux Kernel. What matters is the changes they have made to the kernel, as well as enforcing AVB, SELinux, etc. etc.

“Linux” phones that run modified desktop Linux distros are hugely insecure devices that lack many basic security and hardening features.

CalyxOS is not hardened in any way and is in some ways less secure than stock AOSP. They are also on a hiatus and have discontinued updates: https://discuss.grapheneos.org/d/24791-departure-of-calyx-calyxos-leadership-and-discontinuation-of-calyxos-updates

LineageOS also significantly regresses security compared to barebones AOSP.

• Userdebug builds
• No locked bootloader or verified boot
• Incomplete backports of patches