The main question is: Did MS want an expert PgSQL team to work on Azure PostgreSQL (and may to create a proprietary competitor to Aurora)? Or Did they acquire Citus for its product, to improve and market it further?
It feels like it was the first. If so, it means bad news for Citus product as it will most likely be ignored for a while. That will be really sad, as I don't know any actively supported automated sharding solution for PgSQL other than Citus. There is PostgresXL[1], but there isn't much focus to make it community friendly.