@2beesornot2beesthatisthehoney
Every mine saying his new post has been around for some time. Yes undoubtedly . But it is not that in question it’s the reasons behind the timing of his announcement not the job offer
Presumably because they don't want to gap the post (ideally at all, or for the minimum time) they'll need time to recruit a successor, and for that person to be released from their current post. If it's another secondment then the timings for release are probably faster than having to deal with a notice period.
But even so the 6-7 weeks remaining aren't a huge amount of time and you can't get the ball rolling by advertising a high profile post until the current incumbent says they're leaving