It has HA. Sharding is a different beast which I think Citus has solved.

Can you combine it with some other tool that has sharding ?

I think Citus can do both- sharing and replication. The docs are not 100% clear on this, but they do talk about replication and sharing together.

To use a solution like Stolon would require changes in the core logic to dynamically re-configure whatever sharing plugin you use.