The sycall filter thingy is not in any way comparable with SELinux, and systemd tightly integrates with SELinux as well.
Please don't think that the syscall filter thingy is intended to replace SELinux in any way. Syscall filtering is hardly comparable to what you can express with SELinux policy. This stuff is useful in a few cases however which SELinux doesn't really cover: it's trivial to write for admins, without the need to get the SELinux policy rebuilt and updated, third party software can easily make of this to lock itself down, and it works fine even in systemd user instances, i.e. to lock down individual user services or apps without any system policy updates.
So, if anybody tries to compare this with SELinux, then you are comparing apples and oranges and assuming that there was competition in something where there is no competition.