What is the security hole in systemd?
You haven't answered this.
systemd-tmpfiles exists to delete inactive files in /tmp. That's not a security flaw, that's system maintenance. It's the documented purpose of systemd-tmpfiles and it performs exactly was documented.
The security hole is in snap-confine which does not verify that its own directory is owned by root before mounting it AS ROOT. That's the security hole.
So, again, what is the security hole in systemd?
If you don't have an answer then just say so. Resorting to name calling and trying to frame this as if I'm the irrational one is absurd.