Brent is harping on limitations you hit when trying to use protocol oriented programming in Swift:
But these days we’re smarter: we use protocols. There’s no reason Folder and File should descend from the same class — they’re almost entirely different, and inheritance is a pain to deal with, so we use protocols instead. And we’re happy. It works great. Until you realize that, in Swift, you can’t do this.
I hit something like this yesterday. So no, it’s not just him.