I don't know about anyone else, but I have never suggested an engagement might have been delayed.
It's just the announcement that might be timed.
C'mon - people get engaged when they want, as they do, of course they do. But they can be engaged for an hour, a day, a week or a month (or even longer) before it's public knowledge.
And, if you might potentially get married at Westminster abbey or Windsor castle (or an number of HM's churches I can't name at the moment) because you are 9th or less in line to the throne, then it's an up-in-lights announcement.