Of course relational db can act like a graph db. It's just not as efficient due to how things are stored and queried. Would be great to have a graph db plugin (and I found one https://github.com/apache/age)
Having first class pagerank and shortest path functions in tinkerpop gremlin vs having to roll it yourself with recursive CTEs feels like a graph DB win.
The real question is, is a RDB better at being a graph DB than a graph DB is at being a RDB?