I have to call this out as a very poor comparision. I would love to know the linux kernel commits in 2019 vs CVE's in a percentage, vs systemd commits in 2019 vs CVE's in a percentage (or 2018 or 2017).
Now compare systemd CVE's with OpenRC or runit CVE's or any other init system and then you have an argument. But comparing it to the kernel with hundreds of people commiting changes is weak.
This comment is not defending nor attacking systemd.
A quick look at the list of CVEs in the grandparent comment indicates that there are at least 3 different Assigning CNAs for systemd CVEs: MITRE Corporation, Red Hat, Inc. and Canonical Ltd.
My conclusion is that the systemd project is not its own CNA and therefore not in a position to authoritatively decide which systemd bugs get CVE numbers assigned and which do not.
Compared to the Linux kernel, systemd's CVE rate is very low.