You're talking about the login flow. OP is talking about onboarding.
Go to discord.com right now, click "Open Discord in your browser", enter a random username, and hit enter.
Go to discord.com right now, click "Open Discord in your browser", enter a random username, and hit enter.
But back then there was no official way to browse more or less public servers, which is the case now. I guess to join those it might be good enough in their eyes to just create a temporary account to let users test the waters and hopefully later convert into a proper account.