Likely because you have to run this as a separate db server. The speed improvements come from adjusting data locality on disk so they're probably not able to have columnar & regular data stores in the same PG instance/data files.
Similar to how Infobright is just MySQL but you still can't mix columnar and regular DBs in the same instance, you have to run IB separately.
As Ozgun notes above, the Foreign Data Wrapper APIs just provide a powerful abstraction for what this extension is doing.
Though the DDL to set up a foreign table requires first creating a "server" with CREATE SERVER, this is merely a formality: as in file_fdw the server doesn't actually represent an external process. All I/O occurs within the postgres backend process servicing the query.
Similar to how Infobright is just MySQL but you still can't mix columnar and regular DBs in the same instance, you have to run IB separately.