All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The marriage contract should fulfill the following four conditions: the spouses, the guardian, the witnesses, and the formulation, and once any of those does not exist, the contract will be invalid.
The Hadeeth in question is intended to indicate that there is no difference in the effectiveness of a marriage contract between joking and seriousness if its pillars and conditions are fulfilled. That is clear-cut.
Allaah Knows best.