Of course, the scope is different, the kernel is about 30 times as large as systemd (which is less than I was expecting frankly), but they both match the description "low-level large C language project". Therefore in my mind they should have a similarl density of CVEs per lines of code:

Systemd has 4 open CvEs in ~1M lines of code.

The linux kernel hash 18 open CvEs in ~30M lines of code.