"Do one thing well" is how Unix kernel functions are written, and it's just plain a good idea. Systemd probably follows the first principle internally, many programs do.
Creating production systems out of single-purpose commands connected by pipelines is a different principle, and only works if you keep them pretty simple. It's not a prionciple, but it is how a lot of Unix scripts are written, NOT including the shell that glues the parts together, and not including all the more complex programs, like ed or mail. Systemd doesn't follow the second, because it's more like ed than a text transformation like spell.
A more useful question is whether systemd as a whole does one thing, and does it well. About that, one might usefully discuss whether the Unix principle applies.
[Pipelines were patterned after a subset of "production systems" in early AI, which applied transforms to "produce" new things. They're not the kind of production systems you put on a raised floor]