The point is that he explicitly makes this hard. That's why he is a twat. The issue is that some applications (especially graphical) do get heavily integrated with it which makes it also hard to port them.

Wasn't there also a program that had exactly this goal? Making systemd more modular and portable and so on?

That said, having a common ground to build on is not bad and when you need tight integration. Sometimes you don't really want to do it twice. That's part of why Linux is so successful. It's a monolithic kernel that experienced widespread adoption and driver development. You really, really don't want to redo it all, if you don't absolutely need to. You really don't want to add complexity by making it all super modular either.

Pottering seems to be not exactly the nicest person, but he has a point when he says that one good solution is all that's needed and sometimes even better than two good alternatives.