Yes, apparently politicians and the media were informed at 4.30pm and told to release the information at 6.30pm, which is why Nicholas Witchell said on the news to Huw Edwards: “it’s now six thirty” with stress in the time, and yet the BBC, for some reason, managed to make the statement a few minutes later than other news channels, at 6.35. So that’s why they were wearing black for two hours before the announcement.
Junior BBC journalist Yalda Hakim tweeted the news early, apparently by mistake, and it’s not clear whether the mistake was releasing the correct information too early, or bad information at the time.
When the queen actually died we may never know. The royal family will probably want to maintain that at least some of them were by her side in the afternoon, before she died, whether that is true or not.
I’ve heard that the statement that the queen was “comfortable” is a known euphemism for being sedated with morphine. So it seems likely the queen was unconscious since at least that morning, following a medical event such as a stroke or a heart attack.