I respect them for sticking to this, but I think it's a mistake. It's clear users in general don't care about an app being pure Mac OS controls/SDKs/etc. and their claims about it being a truly superior UX are unfounded (having used both regularly, Figma disproves that heavily). With this, Sketch isn't available on Windows nor the web (until very recently it seems?).
I wouldn't die on that hill.