It's PR fluff -- AFICT it's actually never been true. If it were true, we'd not have a "sort" option for "ls". (EDIT: It would also require a better interchange format than "unstructured text", but that's an aside.)
It's also fuzzy enough that you can always claim it to be true: What is the "one thing", well for systemd[1] the "one thing" is to be a "system daemon" (not an "init system" whatever the definition of that is). I guess you could argue whether it does it well, but I claim that it does -- at least for every single system that I'm running it on.
[1] That's the correct spelling, btw. Not SystemD or any other variation.
Systemd is a whole group of small, factored tools that call to each other. This complaint gets trotted out a lot and I lump it in with the "It is different because it has new features."
So, I haven't tried the exercise myself, so maybe it's easier than I've been lead to believe; but I keep hearing that the problem is that those "small, factored tools" talk to each other and pretty much only each other and that it's very difficult to, eg, rip out and replace just one of them. The argument, as I understand it, is that SystemD is "modular" in theory, but is a monolith in all practical terms.
Note that I'm not arguing for this position so much as restating it to the extent that I understand it. I'm currently using SystemD and while I have my doubts about it, it doesn't cause me any specific pain ATM, so I haven't bothered messing with it.
I think @pmoriarty here just nailed it. It's not necessarily that "SystemD sucks" or that it doesn't do what it proclaims to do. But the fact that it's so monolithic (in practice, if not in principle) and subsumes so much of the rest of the system, and introduces so much coupling, is a very real issue. That switching from SystemD to $SOMETHINGELSE is a huge undertaking is, in and of itself, a problem.
The "Unix Philosophy" of "do one thing and do it well" isn't just a tagline. That philosophy is cherished because it's been proven to be valuable. Replacing init with something that moves away from that is certainly worthy of questioning, if not outright criticism.
Now maybe it's the case that SystemD is really just plain "so much better" that it outweighs all of that. I honestly don't know. But I can understand why people raise these issues.