Surely the distinction is about the existence of a market. There is a market for competing browsers from different vendors which do compatible/interchangeable things that might be selected by different consumers for different reasons. The technical definition isn't the important bit, the regulation exists to preserve that interplay between consumer choice and market innovation.

There is no such consumer-driven market for in-app UI libraries, be they WebKit-based or not. The "consumers" in such a market would be the app vendors, and in fact this is an area well-served by existing products from competing vendors. That's clearly not what the DMA is trying to regulate.