In the demo they clearly have a wysiwyg app allowing them to drag in the icon, so the project is already set up for "live demo". If i'm a non-technical reviewer, like a designer (their target market I believe), I shouldn't be expected to pull down changes and test locally (unless that part can be automated and made super easy to do without fuss).

Figma kind of already does this (visual diff) with some of their tools for designers, although i'm not sure how well that can be integrated into a full fledged web-app who's changes and reviews could be shared by designers and engineers.