Does anyone have memtest86 available at boot console?

I expected to see an option in the first menu at grub boot, where prior nixos iterations are shown,

Perhaps something else is needed, apart from enabling it in the configuration?

boot.loader.grub.memtest86.enable = true;

solved!!

Using,

boot.loader.systemd-boot.memtest86.enable = true;

instead of the grub version is all that was needed.