Stock OS ships security updates on the latest major version. It means that you can only get a given patch level on the same version for a given device. CalyxOS wasn't rebased on Android 12 until fairly recently. As of January 2022 (prior to the Android 12 release), their vendor patch level was 2021-10-01 which means that at the time the OS was roughly behind 3 months in updates.
They were also shipping an outdated version of Chromium (v94) during the same period (this is important since Chromium distributions for both CalyxOS/GrapheneOS are updated through OS updates - and Chromium is whitelisted by the OS to provide the WebView, even if you happen to use another browser). Considering their userbase is privacy/security-conscious, I think they should've been aware they were more vulnerable than stock OS for a while.
Looking at their source code it's also evident CalyxOS is increasingly relying on the LineageOS codebase. Not that it's a bad thing (LineageOS has its own goals but they're not necessarily aligned with security-focused projects), but it's worth noting.
This shouldn't be too hard as the OS is based on AOSP and there are employed Devs working on it as you can read in the Calyx institute's annual report: https://calyxinstitute.org/documents/2021-calyx-annual-repor...