`In Oathbringer, Fabrial's are explained as working by means of trapping a spren. But there are multiple instances of Fabrials requiring Stormlight (Spanreeds, etc.) to work. So which is it?
Fabrials need both a Spren and Stormlight. The spren helps determine what the fabrial does, and the Stormlight is the raw fuel.
We can see something similar with the Knights Radiant themselves:
The Radiant has to swear oaths and bond with a spren. The oaths the swear and the spren they bond determine their Order, and what type of surgebinding they can do. However, even after swearing the oaths, they still need to take in Stormlight to be able to be able to actually surgebind.