Or at least they can't afford a lengthy stalemate and need the troops elsewhere. They also mentioned Chernihiv, which is north-east of Kiev, much closer to the Russian border. If you look at this at a map then it quickly becomes clear that this is a very significant announcement. Yes, it is a withdrawal that is being dishonestly presented as an act of good will, but as a withdrawal it is signals Russia really has given up on Kiev and everywhere west of there. They are already being pushed back to Kherson in the south, away from Odesa. They are also getting nowhere fast trying to take Kharkiv, which means this war is now about Russia taking Mariopul, establishing a land corridor to Crimea and taking the whole of Donetsk and Luhansk.
I think Ukraine wil refuse to make any territorial concessions, and I am beginning to wonder whether Russia might even end up being kicked out of Crimea.