Concerning your edit: I’ve read that, too. That your connection drops, once a new SIM card gets activated. That might take a while, though, or not happen with some carriers or under certain circumstances. As far as i know a cell network is a crazy mix of technology. And from his description it’s not even clear to me when he talks about SMS and when he talks about notification emails or push notifications.
And in other youtubers’ videos I’ve heard they usually seperate their accounts so that it’s not the same account for private stuff on their phone and the important youtube stuff all mixed in the same account.