[ddragon]

Mostly because the end-user application community is not as strong in Julia (though you can create web services/sites and GUIs right now), in the same way the scientific/numerical computing/machine learning/HPC isn't as strong in Swift. Perhaps the point he made was that since his area is ML, then he could contribute better in trying to create a community around that in Swift than he could bring the other domains to Julia up to par with those languages by adding another competing library.

And in terms of the language itself, Julia is very much a general purpose language. Exceptionally general purpose as it's basically a Lisp below the surface, so it can not only support any domain, it can be comparatively easily extended to better support them. Differentiable programming is one such example, as the compiler was not designed for that, but you can just import this functionality as a library. Though the focus is still from desktop to HPC, as opposed to mobile/IoT to desktop (and apple focused) like Swift, which makes so having both languages supporting differentiable programming an overall positive over having only one of these languages.