Correct, in my eyes "software engineering" in the sense of "person who glues libraries together to build systems" should not yet be called an engineering discipline because there isn't yet a rigorous enough theory on why something should be designed one way as opposed to another. We are still in the stage of figuring out best practices and making things more rigorous (maybe the functional programming folks will end up contributing something here, but I don't know enough about category theory).

There are other narrower senses of "software engineer" such as "person who optimizes code" and to me those more qualify as engineering because we not only have a decent enough theoretical background (see Agner Fog's work) but also can experimentally verify things. On the other hand it's a lot harder to quantitatively say if one design is better than another.

I think there's also some work in terms of rigorously modeling concurrent/distributed systems (Lamport's TLA+) work which I'd like to see more of.