From my point of view, I see that because of SB developed using the same engine Cocos2D V3, thats why all what it renders look the same on Mac and iOS. No big differences, no glitches.
If you make changes to that Cocos2D source code, they will affect to the editor and game at the same time(I'm not talking about platform specific code, but just general behaviour). Well, in real life you just need to update your Cocos2D sources after any SB update, good idea was is that SB automatically updates project if it's using old cocos.
So, that's why I think that editor for PureSwift must be developed using PureSwift
Also, I have so many comments and concerns about SB... there are a lot of thing done in a wrong way and they needs to be changed or implemented in a better way in an even more better editor. Hmm.. can I call it SwiftBuilder?