There are caveats to that statement. Semi-formal human social decisions need to be made in order for an ecosystem to agree on a set of interfaces. Some of them are part of Julia Base, which makes it easy, but some of them emerged through a lot of community work, especially in SciML (autodiff interfaces, advanced array interfaces, statistics interfaces, file and serialization IO,). This has not happened yet in the webdev side of things (Pluto, Genie, etc do not share as much) so it is difficult to compose things well there.

Edit: I removed HTTP.jl from the list as it is a lower-level library with a different set of constraints to interoperability.