How to extend brass pipe/conduit that is buried?
You can connect the brass pipe to another brass pipe with a coupling, brass around a pool is the best option as it will not rust. If you would not have said an electrician verified it was brass I would have suggested it was rigid conduit as it looks galvanized to me in the photo. Maintaining the brass will prevent any galvanic corrosion issues , 680.11 allows for rigid pvc but you mentioned pool light. If a wet-niche type light this would explain the comment if they did it right the extra wire. And the use of brass 680.23.8.b.   I would continue the brass for the additional safety it provides for grounding.