"made with Swift" buries the lede a bit here, it's actually using Skia via the Flutter engine under the hood to draw windows, compositing, etc. The Swift part is just the author rewriting parts of the Flutter API surface (not my cup of tea personally, but I try not to miss the forest for the trees). Really cool project!
https://starling.build/why this page explains the design rationale better than the landing page imo (if you can get over the LLM-isms on either)
Flutter for the DE and Swift as the "frontend" for Flutter for widgets instead of Dart – cool work.