"Firefox has open APIs for everything the browser does—a majority of Firefox “application” code is written in JavaScript. Chromium on the other hand does not expose certain areas that are sensitive to Google’s business. The most notable example is the address bar (where Cliqz search lives). Google has no interest in making it easy for others to take control of the address bar; the flow to the default search engine (Google) is protected. Of course one could create new APIs, but that would imply creating them on top of the source-code. This is not a big issue, we have already created a prototype, but then there is the problem of maintaining it."
For the question of "why not call it a browser extension or something" because "browser extension" already means something completely different. It's also a bit wonky to think choice of language should decide if you call something a fork or not in the first place, doubly so considering significant portions of the browser are already written in that language.