I don’t fully understand the relevance of that paragraph, but launchd and systemd both work this way, both for similar reasons
I do believe that launchd came first: https://news.ycombinator.com/item?id=2565780
I do believe that launchd came first: https://news.ycombinator.com/item?id=2565780
Perhaps I misunderstood your comment in interpreting it to say socket activation's an original systemd concept.
I was trying to respond to "Why does systemd have to store file descriptors?" with "That's actually the entire point of its existence"