I don't get this. How do you know that the v231 for that component will work with older versions of everything else? If you do, why not just compile it yourself? If you don't like that, why not upgrade everything to v231?
If it were a separate component, it would have a bunch of ifdefs covering several versions of systemd, possibly with some features disabled if older version doesn't support it. However, that somewhat increases code complexity for developers and systemd devs refuse to do that.
That is the point of 'monolithic' criticism - despite being many binaries, you can't easily just build single one and make that work.