Editor documentation is very sparse, and editor APIs are even more complex then the engine it self so it’s not really surprising. also i think you mean IDetailLayoutBuilder because i can not find any reference of IDetailBuilder in the code
You missing one important thing, IDetailLayoutBuilder has a I prefix, it means it’s not a object but interface containing class that implements it and can’t be created sperately. And by looking on the code it is just set to the function as argument to respond with construction of detail customization. And ot os interface to either body guard the implementing object or to let use different classes as augments.
As for main question i don’t know, by looking on the code it should just work, maybe you doing something wrong.