Nor any semicolons. Also full of nonsense. SWF fundamentally runs action script. The idea that you are going to have a AS3 to C# transpiler and a C# scripting engine AND export SWFs is incongruous. Again, "C# code executed when the playhead enters a keyframe." Now I'd love the oppurinity to learn, but ctrl+f c# here [1] shows 0 results. Though I'm sure I'm just misunderstanding the meaning here ( ≖_≖).
I'm skeptical about SWF export as well, but mostly because I'm not sure of the usecase for it in "flash if flash was built in 2026". I'd imagine even people opening old FLA files would rather have a modern non-plugin-requiring format (or a modern Projector equivalent) for output. Maybe for people still using Scaleform?
only thing I can imagine is that they are building it with Unity, hence the C#. You can compile other languages to SWF bytecode, that's what Haxe is.