Putin used a lot of carrots. Food, fuel, technology, and I believe the prospect of the NK military gaining some measure of modern battlefield EW and drone experience also factored in. Kim himself now seems rather taken with drones, wants NK to develop that production capacity and know-how after seeing what Ukraine has done with them. It's a big leap for a military that is still trying to leave the 60s. Since that generation has passed, NK troops are more experienced at farming than fighting.
Kissing 10k troops goodbye for easier winters and missile/satellite tech is probably an easy call for an imperious, attention seeking, power hungry sociopath like Kim.
A shame, but whatever. Ultimately this joining of the party by NK will benefit Ukraine more than Russia, thanks to South Korean arms prowess. Seoul can give Putin the finger now, effectively saying 'tough shit shorty, we warned you.' 'Anything the people on the other side of our DMZ do, is our affair.'