UL uses a fork of WebCore/JavaScriptCore (the layout engine and JavaScript engine from WebKit). It is indeed modified (been working on it 7 years) and released open-source on GitHub.
From a business standpoint, we were offering paid commercial support for not just Awesomium but all the Chromium-related parts and that surface area had grown to be way too large to be maintainable. For the product to succeed we had to refocus efforts on building a solution we could fully own and met the requirements of our core market-- and that took about 6 years of R&D.