Pixel 9 has very slight security benefits over the 8th generation, but not a gigantic leap compared to 7 to 8.
Pixel 9's modem has additional hardening: https://security.googleblog.com/2024/10/pixel-proactive-security-cellular-modems.html?m=1
Because the higher Linux kernel version, some Linux kernel security features like memory structure randomisation can be used. This is exclusively enabled in GrapheneOS:
An explanation on that is here: https://stacker.news/items/670170#randstruct-kernel-61-and-above