They're completely orthogonal tools.
And no, it was hardly a "big reason" why distros weren't interoperable. Initscripts were rarely supplied by upstream vendors and frequently written downstream. Then many other people didn't use them at all, but wrote runscripts for some flavor of daemontools in about 4 lines or something.
The main reason has always boiled down to the toolchain. One proposed way to get around it was fat ELF binaries, but that effort got metaphorically curbstomped by the Linux community. Universal package managers aren't supported for political/branding reasons.
In any event, systemd doesn't help with this.