Thanks for the reply. "Native Graph" here means the system (including the storage and query engine) is designed around the Graph data structure. The opposite part of the "Native Graph" is usually called "multi-mode" databases. In other systems, the storage is designed either as tables, or as some other data structures. They only provide a Graph query interface to simulate the graph query engine. But behind the scene, they are still doing the SQL (or whatever) queries.
In Nebula, data are stored in a way so that getting all neighbors is actually a sequential read