Maybe it's a cultural thing. You usually stipulate a leaving date when you hand in your notice or you resign with immediate effect.
Not in the UK you don’t. You confirm to your employer you wish to leave, and to do so you must provide them with your notice period which is written into your contract.
As I said before, a notice period works both ways — if the shoe was on the other foot and Southampton wanted to sack him, they’d have to give him 12 months notice before doing so, or pay him for his full notice and let him go.
Something that a lot are missing here — we aren’t talking about ordinary junior staff members who normally have a 1-3 month notice period — these are senior executive level employees at director status of huge companies. Large notice periods, garden leave provisions, and restrictive covenants preventing staff and information poaching is very commonplace in this market.
Where are you from, out of interest?
Edit: I think the “resigned” wording has come from dramatic journalists like Romano and has since been reigned in by other tweets.