D&D Beyond is being developed by Curse, and Curse is part of the Twitch family. Curse uses Twitch accounts now. I'll try to channel Socrates here:
D&D Beyond accounts are Curse accounts
Curse accounts are Twitch accounts
Therefore, D&D Beyond accounts are Twitch accounts
So, there's no difference in logging in with your Twitch account and logging in with a DDB-specific account - they are one in the same. We appreciate the feedback on this.