WebKit is a rendering engine, not a browser. It's a bit like saying "that isn't original art because you used Earth physics that govern how paint flows and dries.
I dunno. It's more like saying "that isn't your original art because you didn't paint it, only provided the frame". A pretty elaborate frame, yes, but only the frame, not the painting.
I think this is underselling the browser UX. In many ways, Safari, Orion, Chrome, Brave, Opera, are all very different products despite all sharing the WebKit lineage.
The particular choice of the marketing phrase "from the ground up" sort of invites this kind of commentary on HN. This is hardly the first WebKit-based browser, so any commenter specifically holding their breath for a new, independent rendering and layout engine or JS engine might feel inclined to roll their eyes and snark.