Andreas Kling talked about it. It boils down to C++ interop sucks (no surprise for lang made by Apple), ecosystem is tiny, Rust works well enough with LLMs.

https://youtu.be/DbHjKi_jASY

Swift is one of the few toolchains that actually has some kind of builtin C++ interop, alongside Objective-C++, D, .NET (via C++/CLI), Carbon (eventually).

He just wanted to use LLMs for coding, and not enough training data on Swift code exists for his use case. Admitting to that would be rather silly, so here we have this sentiment now exist rent free in people's brains.

The C++ interop of Swift is perfectly fine, to such a degree that FoundationDB is now using it effectively alongside its C++ origins.