This is a great point, and you're not wrong. This was a pretty unique case where I had a really clear idea in my head of what this would require from the get-go, and it was also meant to be a pretty quick, experimental little project. It seems to have struck a chord with some folks, so now I get to find out if my off-the-cuff approach is going to come back to haunt me soon.
As for SwiftUI, I think it'll be a slow and steady process for most of us. I've been a UIKit developer for the better part of a decade now, so I still feel most comfortable in that domain, but I think most folks acknowledge SwiftUI is the future. Although it's still a bit immature, I'm hoping to start shipping bits of SwiftUI here and there soon.
As a counterpoint, SwiftUI is easier to grok to me (a student who's done a lot of web work in React) than UIKit (especially storyboards and the constraints system, but maybe I haven't made enough of an effort to learn it). HStacks and VStacks are very similar to flexbox layout.
Are iOS devs still shying away from SwiftUI for now?
Also, I find it interesting to start coding on day 1 with just a vague idea in your mind. It's probably my UX influence, but I wouldn't dare do that anymore these days. In the least, I want to hash out my domain/data model, no matter how simple the app might be.