It seems like the argument being made here is that perhaps the systemd developers are afraid / unable to innovate in the kernel, which is in fact the overall system-wide daemon and parent to all processes.
Note that one of the stated goals for systemd is to make good use of all the new fancy features are already available in the Linux kernel but not many people use because they are not portable (eg. control groups). Also note that some cool features have already been merged in the Linux the kernel after the systemd developers created them (eg. memfd).
Heck, he has more than once chewed out one of the principal systemd devs (Kay Sievers) for violating central rules of kernel development.
Edit: phrasing