They don't maintain a separate Chromium codebase, nor do they refer to it as a fork anywhere on GitHub. They do refer to it as a customised Chromium, which I think is a far more accurate description:
> Brave Core is a set of changes, APIs, and scripts used for customizing Chromium to make the Brave browser.
I also think of Chrome as a customised Chromium, not a fork of Chromium.