>The string processing is powerful, but inconvenient when you want to do things like indexing by offsets or ranges, due to Unicode semantics. (This is probably a good thing in general.)

This is being too generous to Swift's poorly designed String API. The author gets into it immediately after the quote with an Array<Character> workaround, regex issues, and later Substring pain. It's not a fatal flaw, a language backed by one of the richest companies in the world can have few fatal flaws, but AoC in particular shines a light on it.

I really like Swift as an application/games language but I think it unlikely it can ever escape that domain.

I'm curious, in what niches are people using Swift for new applications these days? I've enjoyed working with Swift in the past (albeit in very limited capacities), but I haven't personally come across any Swift-based initiatives in a while. I had high hopes for Swift for TensorFlow, but it was ultimately killed off.