My impression is that it refers to the multidisiplinary aspect of it. It's not enough to know orbital mechanics, you also need to understand - control theory, thermodynamics, material science (can't let the thing burn up in the atmosphere on re-entry), structural science (can't let the thing shake itself apart), chemistry (fuel), fluid flow (both for the engine and for launch/reentry), telecommunications, etc. You have to combine all of these fields while designing under very difficult restraints regarding weight, cost, and safety.
There aren't too many successful commercial developers of rockets. My last company (which I decline to name) was pursuing this when I worked there. I wasn't involved, but I did know some of the people working on it. The rocket blew up on launch, destroying a multimillion dollar payload. It _ain't_ easy.
Yes, I am conflating engineering with science, and I don't care about that. It's an expression.