It literally just checks whether navigator.brave.isBrave(). The following Greasemonkey script is sufficient to fool it:
// ==UserScript==
// @name Very brave
// @match https://together.brave.com/*
// @grant none
// ==/UserScript==
navigator.brave = { isBrave: () => true };
Edit: I guess it's strategy, pushing more people to use Brave browser, rather than an actual limitation of the software.