This tweet is speculation, there is no evidence presented in the thread that it is A/B testing. The message says "Sign in to confirm you're not a bot." Given t

This tweet is speculation, there is no evidence presented in the thread that it is A/B testing. The message says "Sign in to confirm you're not a bot." Given that this message is being presented to Cobalt, an automated downloader tool, it seems likely that YouTube has detected bot activity and is throwing up barriers to stop botting. That is very different than the implication that YouTube is testing out the idea of blocking human users from watching while logged out.