@kristoffer.carlsson - thanks for the link to the presentation and for the explanation.
I suppose that whenever I have a mutable struct Foo ... end, then a Vector{Foo} will actually be implemented as an array of pointers to heap-allocated Foo objects rather than directly as an array of consecutive Foo objects. That would be consistent with Java, Python etc. and makes sense for a GC language. My only surprise is that such a mutable struct was chosen to implement Atomic{T} - definitely not what I expected given the zero-cost implementation in C/C++/Rust/etc.
Hopefully @atomic would allow for this zero-cost abstraction (though that wasn’t explicitly stated in the talk).
At least now all is clear - many thanks!