They don't use Electron. Their website states that they don't plan to release Windows or Linux versions because of the large amount of native MacOS API's they use making it very time consuming and costly to make a cross-platform version of the app.
I guess there's a market for a windows/linux version of this app then? (tho' maybe not as large, given pro-designer tendencies towards macs)
There's a third-party app for Windows called Lunacy [1] that can read/write Sketch files, so there must be a market large enough to make that worthwhile.
I wonder how much apps like Sketch and Pixelmator rely on CoreImage to do the heavy lifting. They all seemed to appear after its introduction.
Sketch is mostly vector-based...
Quartz is mostly vector-based as well... OS X has very good support for vector graphics, it's rendering engine is basically a version of PDF-style rendering (like old NeXT used "display postscript").