Can't NoSQL DB ensure consistency if it is residing in a single server?
Yes, a write to a single instance, distributed database will be immediately consistent.  You likely still won't have ACID, as each NoSQL variant has its own ways of handling (or not) atomicity.
Although, I have to wonder what the goal would be here.  The main feature of a NoSQL database is its ability to support partition tolerance...which is lost in a single instance configuration.  I can't think of many scenarios like that where I wouldn't just run Postgres.