No story. They are outsource team. They have contracted with IOHK to deliver Cardano SL. They have delivered Cardano SL. Contract ended. Now IOHK contracted with some other team that has engineers of different specialisation.
Afterwards Serokell said that they are big fans of Cardano themselves, and gonna support its development from community side of things (building dApps and infrastructure). I think they are great team and gonna be one of the active proposers for the future treasury and gonna receive good votes and funds to make Cardano even better.
As their token of love and appreciation for the project, they are launching the staking pool: