Small programs communicating over pipes via plain text was/is the implementation. The philosophy is more general [1]. This one is more or less how I think of it:
"Even though the UNIX system introduces a number of innovative programs and techniques, no single program or idea makes it work well. Instead, what makes it effective is the approach to programming, a philosophy of using the computer. Although that philosophy can't be written down in a single sentence, at its heart is the idea that the power of a system comes more from the relationships among programs than from the programs themselves. Many UNIX programs do quite trivial things in isolation, but, combined with other programs, become general and useful tools."
"Medium sized" (systemd alone is around a million lines) C-programs are extremely difficult to debug or modify if you're not intimately familiar with the codebase. The code in them can't typically be used for any other purposes. Sysvinit as a program is very small, but the point is the protocol that allows multiple different kinds of programs to co-operate. Being able to glue things together with shell and Perl is a feature, not a bug.
Journalctl is a good example of violating the UNIX philosophy. It contains its own binary format, its own grep, its own head and tail, etc. With /var/log files you don't have separate cat, grep or tail to examine files and directory listings. That is the UNIX point. And this is what I mean by general purpose.
sysvinit had/has plenty of problems and systemd typically works better at least nowadays. I'm not singing any praises of sysvinit. But e.g. OpenRC manages initing without a million lines of code and taking control of more or less the whole OS.