"Scale your graphs with unlimited vertices and edges, and more than 100,000 queries per second for the most demanding applications. Storage scaling of up to 128Tib per cluster and read scaling with up to 15 replicas per cluster."
That isn't real horizontal scaling, they are just doing a single vertically scaled writer and read only replicas, just like they do with RDS. It probably is using the same infrastructure for it.
From the marketing page below:
"Scale your graphs with unlimited vertices and edges, and more than 100,000 queries per second for the most demanding applications. Storage scaling of up to 128Tib per cluster and read scaling with up to 15 replicas per cluster."
https://aws.amazon.com/neptune/