This seems to be an incrmental rollout, however, as not all users have reported getting a notification

Some MacBook users have also reported receiving a similar notification

I checked with both iOS and MacBook users asking them to restart their laptop, set the date/time to April 20th at 23:59 and within a minute, it sprung forward to April 21st at 01:00

So this is undoubtedly 2023b and nothing else