The qualifiers in the original sentence are I believe important; it's not that Swift generates more efficient, compact code than C++, it's that it's far easier to do so in Swift and that Swift has far fewer ways for junior to mid-level devs to shoot themselves in the foot. Swift I believe also has a much richer set of standard libraries than C++, which removes lots of need for pulling in third party code - e.g. Swift has a built in for handling URLs, I'm pretty sure C++ still doesn't.
If you're writing high frequency trading code, C++ is definitely where it's at. If you're writing run of the mill mobile apps, you'll get far more done with fewer bugs using Swift. Automatic reference counting is a massive improvement manual memory management - less code and less bugs, and wins over GC on performance, peaks, efficiency, etc.
Even on mobile, there are cases where C++ makes sense. It's all about using the right tool for the right job. The original article from Dropbox makes a number of very good points.
(Aside: I'm unsure if by 'compact' the OP meant 'small compiled binary' or 'without unnecessary boiler plate in the source code'.)