"... eventually act as the single home for low-level system interfaces for all supported Swift platforms"
"... mainly covers just a few file-handling APIs"
Because you can't find/write a C lib for THAT?
"...a problem is that SwiftUI"
The part from above points out not the problem with Swift, but Apples thinking in general. Apple seems to ignore absolutely everything non-Apple to the point where they have no idea on what makes the competition attractive. In this case, they're porting a piece of something in solitude without regards to how it has become attractive in even its native environment! That just reeks of "We've been isolationists so long we don't know how the rest of the world works anymore", or something like that. Again though, here's also the suspicious advertisement of porting the easiest things to port (stdio)... no C programmer is impressed with confidence by that.
I dunno. I'm not an active IT professional anymore, but I've LOOKED at Swift and thought that if I wasn't "All-In" with Apple, then there's no point as it seems like an off shoot of their Objective-C. However, if they really want to port something useful, they should port their actual GUI toolkit first. This article's author is dead on here with the SwiftUI comment. Porting something _ONLY_ in the middle doesn't even give people a chance to use it, it's basically like making proprietary drill bits without a drill to use them. I only keep Apple stuff on my radar in hope of their GUI toolkit, everything else to me is just platform specific fill-in.