During a brief period during the beginning prep phase of Proving Grounds before Proving Grounds takes over and removes most of your UI, you still retain access to your Global Chat window.
However, it is a trick. While you can type into the Global Chat to try to have a conversation as normal, the game does not like that at all.
Any key presses you enter into the Global Chat frame rather than registering as just chat also get registered as UI hotkey presses. What I mean by this is that if you, for example, tried to type “jump” at this time, it would type “jump” into the global chat window like you were chatting but it would simultaneously also flip out and open your quest journal, friends list, map, and character profile page.